I usually play a benevolent hyper-consciousness machine race, or some variant on the maybe-not-entirely-horrible synthetic life theme. Sadly, they're not going to be in.
I also sometimes play a slightly more chaotic insect hivemind theme, when I feel like paving a path of destruction and ignoring my better (moral) judgement. If that theme is done justice in the game I happen to be playing, or at least the game is open enough to allow that interpretation. But that isn't really going to be in Stellaris either? I don't think? At least not to the extent that I'd like.
So I have a feeling my first game will probably be humans. Though probably something a bit less straight-forward than empires and federations. I am thinking a reasonably benevolent corporate dictatorship, where a single corporation has managed to dominate space travel, and runs their colonies like subsidiaries.
Anyway, most of the time I don't go on a rampage, I'm more interested in trade, diplomacy, expansion, and playing defensively.