This can be tricky to determine, depending on whether we're basing it on gameplay, narrative, some mix of the two...
I'd say that Jack is a definite contender. She doesn't seem particularly focused in her application of force, but the devastation that follows her on Purgatory is impressive. Aria strikes me as similar to Jack in being hugely powerful, but unfocused and impulsive. Aria, being tempered by age and experience, I think would take an edge over Jack. Samara seems as though her rigorous training, centuries of experience, and apparent life of unending conflict would place her mighty high on the list, moreso than a hypothetical asari matriarch who, though likely powerful, may not have the same level of "live-fire practice" as a Justicar.
Liara can be impressive gameplay-wise, but I can't help but feel she'd be just below the threshhold for true biotic greatness: very, very good, but not really awe-inspiring. Same for Miranda, despite my personal love of KO'ing husks with Slam. Kaidan, I regret to say, I don't have enough experience with to make a qualified judgement. The two that seem to be overlooked, though, are Wrex and Shepard. Krogan supposedly produce powerful biotics, and Wrex shares the centuries of experience that help boost Samara and Aria in my estimation. Add in the life of conflict, hardy physiology, and tactical/battle knowledge, and I would argue Wrex is a very strong choice for any top three.
Shepard is more difficult because obviously every Shepard is different, but from my own experience playing as an adept, I feel my Shep could hold her own against nearly all of the other encountered biotics. The speed and force at which an adept Shepard can dole out destruction is truly impressive, especially at high levels. Its that recharge time that really kills some of the biotic squadmates gameplay-wise, something that doesn't seem to hamper Shepard as much. I would argue that the things truly holding Shepard back are his/her relative youth, and the relative newness of humanity to the whole biotic thing. Alliance biotic programs have clearly had some growing pains in the past, and probably have plenty to room for improvment.
This has been a ridiculously longwinded way of saying:
1. Wrex/Samara
2. Aria/Jack
3. Shepard
Wish we'd seen a bit more of Saren's biotic abilities. The relationship between turians and their biotics seems quite interesting from what little we've learned from the Codex and Nyreen...