The title of this post shouldn't be a huge surprise; after all, the whole point of the leader role is party support. As such, the composition of the rest of the party is more important for leaders than it is for other roles. It's imperative that any leader knows the capabilities of his or her allies and chooses powers accordingly, but to achieve true party synergy the party and leader must quite literally be built for each other.
Building an Optimal Party
At this point I would like to make a disclaimer: I am not advocating one way of playing D&D as "better" than any other. While most of this post should be of at least some interest to players in general, this section caters to the player (or more likely, group) that enjoys not only character optimization, but party building and optimizing synergy as well.
The most extreme strategy, but the one that will likely yield the highest degree of synergy, is to choose a leader class and then build the rest of the party around it. This is unlikely to happen in most casual games, since it puts a severe restriction on the character options of the players. A group can go about this in two ways: 1) go along with whatever the leader player chooses, or 2) cooperatively discuss how the group would like the party to function, and then determine which leader class will be chosen as a group. Number 1 only works if the leader player is really determined to play a specific build, but the rest of the players are non-commital. Alternatively, one player can build an entire party just as a theoretical exercise, and then possibly play-test it (or play a nice, embarrassing game of D&D Solitaire). Number 2 combines input from all of the players and thus it is more likely to occur in an actual game, but it still requires that the whole group be interested in party optimization (or, as the case may be, some players may be uninterested in the exercise, but happen to want to play a class that fits with the group's overall concept).
A less extreme approach would be for the leader player to wait until everyone else has come up with their character concept. The player then chooses a leader class which complements the existing party best. The leader will likely want to ask each of the other players about specific details of their build (for example, "will you put a lot of resources into improving your Barbarian's charges?"). This way the leader will know what to build their character around. Note that for a new player that's joining an existing group (and happens to be the leader), this is the best way to achieve synergy. In fact, this situation may be even more ideal than building the leader and the rest of the party simultaneously, as an existing group already knows how their characters function from experience rather than theory, or intent.
Finally, there's the inevitable situation wherein all of the players have a specific character concept in mind, and they're not necessarily thinking about party synergy. This probably happens in the majority of cases. Obviously some leader classes (or builds within a class) are better at certain functions (to be discussed later) than others, but only rarely will a leader be detrimentally incompatible to a party (the only example that comes to mind is an Eagle Shaman in a party of all melee characters). Regardless of how optimally your leader class fits with the rest of the group, you can always choose powers that synergize with the party, despite the fact that they may not be typical of your build.
Examples: These are meant to spark ideas, and do not necessarily represent a specific party composition.
An Eagle Shaman with a Sorcerer, bow Ranger, Fighter, and one other melee ally in the party (both the Sorcerer and the Ranger have good RBA's that the Shaman can exploit. The Fighter is usually the stickiest Defender, and so will have less trouble keeping enemies away from the rest of the range-heavy party. Another melee character will help fill the front lines with the Fighter).
A World Speaker Shaman with a Warden and several squishies in the party (the Warden isn't very sticky, and the Shaman's Spirit Companion (SC) can help keep enemies near the Warden and away from the squishies).
A Valorous Bard with low AC/HP melee strikers (those Temp. HP will keep them standing).
A Tactical Warlord with a Rageblood Barbarian, Paladin, and other melee allies with good MBA's (the TacLord enhances the strong melee presence of his allies, and puts Commander's Strike to devastating use via the Barbarian. Weak healing is made up for by the Paladin's Lay on Hands).
Broadly speaking, a leader is a support character. As such, you make the party better at overcoming encounters with minimal expenditure of resources. This may be accomplished by patching gaps or weaknesses of the party, or by enhancing the strengths of your allies.
Most leader powers can be categorized as either rescue powers (namely healing and granting saves) or enhancement powers (buffs/debuffs). Leaders often have powers that grant extra movement, and these can fall into either category. Sliding an injured ally away from the troll that's about to bash in his head is an obvious example of rescue, whereas sliding an ally into a flanking position is an enhancement of tactical position; specifically, the flanked enemy is de-buffed (grants combat advantage), resulting in a net +2 to-hit for the flanking allies (which may include a damage bonus, i.e. a Rogue's sneak attack or Druid's claw gloves). Rescue powers are defensive by nature, whereas enhancement powers may be either defensive (granting an ally temporary HP, for example) or offensive (granting an ally a damage bonus). Note that some offensive powers are effective for any party member (a to-hit bonus, or a flat damage bonus) whereas others are more situational (granting an extra melee basic attack is most effective on an ally that has a strong melee basic attack). The major difference between the two categories is that rescue powers are reactive, and enhancement powers are proactive. Any good leader should have some of each in their repertoire.
In a vacuum, offensive enhancement powers are the best choice. These consist of to-hit bonuses, damage bonuses, or granting extra attacks. They'll theoretically result in the party killing enemies faster, and since dead enemies don't do damage the party will lose fewer surges per encounter. Since fights are shorter and party damage is higher, the rate that the party needs to spend Daily powers will likely decrease. The overall mechanical effect is a longer adventuring day.
Of course there's a difference between theorizing on paper and actual gameplay. Sometimes the party makes a series of really unlucky rolls, despite the leader's buffs. Sometimes it's not readily apparent which enemy in an encounter is the most dangerous. Sometimes a foe has debilitating status effects. And sometimes the enemies simply take advantage of the terrain better. In all of these instances, something goes wrong and the party is placed on the defensive. It's the leader's job to either protect the party enough to get them through the fight, or to at least get everyone on their feet so that a retreat can be made. This is where rescue powers come into play (fortunately, all leader classes have their 2/enc. minor action heals, so even the most offensive leader has something to fall back on). This is also where an important line is drawn between theory and actual play; in theory, an offensive leader can help the party overcome the majority of encounters more quickly. However, the majority of encounters aren't life-threatening, and the party would probably be able to slog through them regardless. It's the tough encounters where the PC's lives are really on the line, and while good offensive buffs will certainly help in these encounters, the fact is the party is outnumbered and/or outmatched. Allies will fall, status effects will often be inflicted. Backing the striker up with a heavy defense will probably be more effective in the long run than giving everyone a small damage bonus.
In the grand scheme of things (and because of the unpredictable nature of actual play), there is no overall strongest or most optimal function that a leader should specialize in. The most important thing is to make sure that your abilities complement the party. If half of your allies are defenders, including a Paladin and a Life Warden, you won't need as many rescue powers. Conversely, if you have a reckless Rogue that likes to charge into melee and flank, while rarely (if ever) sniping from safety, you'll probably need a lot of rescue powers. Which brings me to my final point: while selecting powers based on what classes are in your party will go a long way toward ensuring that you can effectively support your party, you must also consider the other players. Cautious players might feel more comfortable if they know you have plenty of heals or defensive buffs for them. Reckless players might need these things, whether or not they actively tell you. Power gamers will probably appreciate an even bigger damage boost, while secretely you know that some of the credit for their kills is yours.