I'm not even sure that question can be answered, but still: my plan is to have one of each class at lvl 80 eventually and just faf around doing heroics and some 10 man stuff, switching chars from week to week, so ideally they're all used in equal amounts, no one main. I removed JC from my Shaman but was thinking of taking it again on one of the chars, but which one? Admittedly it's a service but would it be a better idea to have JC on a hybrid (druid/pally possibly) or something simpler like a hunter? (I'm not even sure if you can answer this for BC, let alone WLK, but any opinions welcome^^)
Like Enchanting,I don't think just one class benefits from JC, it is honestly really spread out what a JC can offer.
I do agree that a hybrid would most likely see a bit more benefit, especially if you plan on switching specs often with them. For example swapping your druid between Feral and Resto, you can make the trinkets that boost each spec. Lets face it, getting good trinkets can be a real pain.
Also, consider the jeweler only gems. The BoP gems are generally much stronger then blues, in BC in line with purple gems. With WotLK, you can have three equipped at a time, making for some truly intresting prospects. Using the druid example again, you can stack up on sta, and agi gems for your tanking set; while you boost your spell power, and mana regen for a healing set.
I agree with C Rage, one class doesn't really benefit more from it than another one would.
Personally, I'm an enhancement shaman and I'm LW / JC. LW for drum rotation, Carapace of Sun and Shadow and JC for the the Hard Khorium Choker.
I guess it really just depends on what you want out of it. The BoP gems are stronger, but you generally (for PvE at least) end up using the regular epic gems anyway. I personally like the idea of a JC as a tank, because some of the tank trinkets you can get from JC (for bc anyway) are some of the best trinkets you can get, if you don't have access to BT or SWP, but I suppose it's that way for every class =)
as a tank, the defnse rating trinkets are awesome, but PvP wise, i say rogues, with the panther trinket, benefits the most. massive boost to AP, and an increase to stealth level. when you are in an arena, or a battleground, that extra stealth really can make a difference in rogue vs rogue fights.