The top races are quite varied in the games I've played on a range of maps and from normal to crippling. With the higher economic bonuses I'd say the Drengin have gotten much better, though I can't reliably make that assumption on ~8 or so full games.

I've seen the Altarians, Torians, Krox, Iconians, Yor, Arceans and finally the Drengin pull ahead to be the biggest threats. Last game was the first time for me the Drengin wasn't the whelp of the galaxy and the first eliminated. Of course last game was on gigantic and the Drengin had about 1/3 of the galaxy to themselves at the start. The Terrans have always gotten a bum rap in my games so far, but I do often play with tech trading off, or play as Terrans myself. I would say the Yor and the Altarians have been my most consistently strong enemies, though given the right opportunity / placement on the map every race has pulled ahead mid-late game. That's when I attack them, even if they are my allies because I don't like being shown up like that.