The toughest opponents that I have played with over my time in ESF:
Alpha 2.0 - Apes and their beams, some other peole who's names I can't recall
Beta 1.0 - Bi0 Brolli, Nightshade, GoldenSaiyan
Beta 1.1 - Lynx, Kenshin (my |::Z::| comrades): the only two people who had the same aggressive playstyle as me.
Beta 1.2 - Optional, the way he combines usage of kiblobs at every stage of melee is great.
Apes are too easy to play against in 1.2, guess they still need to grasp the new system. Alea is pretty good as well. Apart from that, I haven't played anyone else worth mentioning.
Let's see what 1.2.1 brings.