What is the best party enhancing D&D 4ED class and spec. ?

Certainly any leader type...enhancing the party's effectiveness is what they're supposed to do after all. Some of the more effective ones imo:

Bard - most flexible...unlimited multiclassing allows the most customization; main downside is that the more you plan to multiclass, the less likely you can afford to have really high stats

Cleric - best healer, also most effective if undead are a frequent threat; downside is they tend to be less effective in combat

Warlord - best fighter...besides significant combat capabilities of their own, often gives other heavy hitters in the party additional chances to strike; downside is that they have nothing in the way of offensive ranged powers

Shaman - main benefit here is the capability to heal two PCs at once, plus some of the spirit powers are pretty nifty in assisting other party members offensively or defensively (depending on type); downside is that they are probably the least effective in combat (mitigated in that the spirit companion is usually the one in the thick of things)