I was always of the impression that at least Sotha achieved CHIM as well. Is it just me, or has the way we talk about that concept completely changed in the last few years? It was never this ubiquitous before.
Really, the evidence points towards Almalexia being the most powerful, in terms of battle, of the three. An easy way out would be to say Vehk with Prep-Time, of course. Wasn't the whole point of the Tribunal that they were three equal Gods? Mastery, Mercy, Mystery. Or, more darkly, Mephela (You are mastered through hidden webs), Boethiah (Mercy bestowed on you only in relation to deadly overthrow) and Azura (Mysterious through blinding excess in knowledge). In their own spheres, I don't think any of the three could touch the other, if looking at the Tribune in their prime. Saying Vivec could easily match Almalexia in battle is akin to saying he knew more about Clockwork than Sotha Sil. While all three of them were warriors to some extent, Almalexia seems the most fearsome.