Last year Komodo beat Stockfish in the TCEC final, but so far this year Stockfish has been the engine to beat. In the semi-final stage Stockfish won by a decent margin, with Komodo coming in clear second. Critter was the whipping boy, finishing with only 14 points out of 48, but the surprise was Houdini's distant third place, struggling to reach 50%.
The final will be a 64-game monster; for now Stockfish has an early 2-1 lead, having won game 1 and drawn the next two. In the previous stage they went even against each other and had similar plus scores against Houdini; the difference maker was Stockfish's brutalization of Critter: 10-0 with six draws.
You can watch the ever-live action here, and in a week or two we'll know which 3100+ rated beast will be the champion (until the next build, when the previous results may be meaningless).