There are a high number of variables in this equation, but here are a few answers:
For clerics in 3.0: Persistant Spell allowed a cleric to put up multiple powerful buff spells and leave them up all day. This made him a better fighter than any of the fighting classes, by far.
For wizards and sorcerers in 3.0: Empower Spell being thrown on buffing spells or damage spells a few times over made some pretty impressive totals. An honorable mention goes to quicken spell, though sorcerers needed to grab a second feat (probably arcane preparation from T&B) to use it.
I'll assume that people playing 3.5 are only using things produced after the core books came out ...
For clerics in 3.5: Quicken spell is a very useful feat. Quickened spells do not provoke AoOs, so they can be used in melee combat with no real risk. Being able to cast a quickened divine favor right before combat begins can make a *huge* difference in a bettle, as can a quickened resist elements or some form of travel spell. In addition, being able to use up those high level spell slots while still doing a full attack can be *nice*.
For wizards in 3.5: Quicken is again the king. Two spells per round is a great benefit, even if one is of relatively low level in effect. If you plan out your spells wisely, you'll have plenty to spend. Even if you run out of spells, you can still use scrolls, wands or staffs, so allocating a good number for quickened spells is really nice. Quicken goes off the charts for melee wizards (elrtich knight, spel sword, etc ...) that use up their standard or full round actions on melee attacks.
For sorcerers in 3.5: The psuedo quicken feat in the miniatures handbook is a real candidate, but I like empower spell and I hate the mini handbook stuff

. You get to use empower on so many spells as a sorcerer that it is hard to pass up. Empowered fireballs as 5th level spells will be a trend for most sorcerers.
For druids in 3.5: Natural spell is an obvious choice, but it is not really a metamagic feat, so I'll go with quicken spell. Being able to drop spells in the middle of combat with no fear of AoOs while full attacking as an animal growthed creature ... Yesh. The druid can make such great use of his full attack routine that his spells often go to waste or are relegated to use before combat or outside combat.