Grahf (Xenogears): OR Weltal is insanely powerful and he has the ability to beat the living crap out of a huge gear WITH HIS BARE HANDS.
Magus (Chrono Trigger): His magical prowess is unmatched, making him an invaluable asset to the team. I envision him hovering above OR Weltal, as I think he and Grahf'd get along.
Albedo (Xenosaga): Albedo's insanity is a beauty to behold, and his regenerative powers ensure that he keeps coming, not to mention the psychological aspect of him tearing off his own head while it laughs maniacally. Plus, as an added bonus, Albedo pilots an E.S. which could prove useful in a pinch.
Laharl (Disgaea: Hour of Darkness): When properly equipped, our spunky Lord of the Netherworld is virtually unstoppable. Taking into account that he could learn every spell, and max out their levels, and himself at level 9999 with the best weapons and armor available, well, let's just say he'll be wiping out pretty much anybody in his path with no difficulty at all.

Honorable mentions:

Cidalfus Orlandu, aka the Thunder God Cid (Final Fantasy Tactics): Orlandu's powerful sword skills are a force to be feared, but given the respective power of the others involved, it simply doesn't stack up.
chaos (Xenosaga): I admit chaos is one of my favorite characters, and he does seem to possess a hidden power that could potentially be quite good. However, until I see more of this power manifest (I have not yet beaten Jenseits von Gut und Bosse) I cannot properly judge. If so, he and the E.S. Asher would be replacing Albedo and his own E.S.
Kefka (Final Fantasy VI): It was a tough decision between Albedo and Kefka, but Albedo won out in the end because of two factors, his regeneration, and his E.S. Kekfa's angelic form is most certainly mighty, and his insanity is equitable with Albedo's, but in the end, Albedo won out. I hate to say this because Albedo's character is so enjoyable, but he's probably the first to go on that team.
Fei Fong Wong/Id (Xenogears): The decision between Grahf and Fei was a tough one, and I'm still actually debating over this and may replace Grahf with Fei eventually. Weltal-Id and the Xenogear are both powerful machines. Fei is decent on foot, Id being considerably more so, but having seen Grahf's performance on foot against gears, I am inclined to say that his physical capabilities are greater. However, as we never are able to see Fei fight in the same situation later, with his full power unlocked, it is most certainly a tough call.