The warlocks.

Signed,
The warlocks.

I would think Elemental shamans. They have at least one guaranteed crit every 8 seconds (Lava Burst), so it's pretty much 100% uptime for your side of the FM buff.

There is a post on EJ that I found linked on the forums once, can't remember where it went though. This is what I remember of it:

Shadow Priest is actually the #1 answer when there is no other mage, secondarily destro locks. In order of best use: Fire/Frostfire Mages get the most benefit, then spriest and destro locks. After that demo, afflic, moonkin, arcane mage, and then finally ele shaman (benefits the least because of built in 100% crit mechanic with lava burst).

If there are no caster dps, then you can use FM on the following dps:
Enhance shaman, ret pally, any rogue (although mut rogues are better than combat or sub).

I'm not sure if FM works on feral druids, but I don't believe it does (although I could be wrong, seeing as how some Feral druids use Black Magic as their weapon enchant now). It definitely does not affect hunters and warriors.

If all else fails throw it on a healer. And make SURE you throw it on your holy pally/resto shaman that is going in the portals on Dreamwalker. :)

I'm not sure how tehnically correct it is, but my FM priority list goes something like this:

1. another mage (reciprocity!)
2. warlock
3. boomkin
4. holy paladin
5. shadow priest
6. elemental shaman
7. ...anybody else who uses mana who isn't a hunter.

It also depends on the group I'm in - if I know our boomkin isn't all that great at staying out of fire, I'll drop it on our holy pally, etc.