I use 700x25 Armadillos. I like them, but I know a lot of people who swear that 28s are the best balance of speed and comfort for an "all purpose" bike.

I don't think, personally, I'd commute on anything smaller than a 25. And since this is a commute, not a race, I'm more concerned with all condition durability mixed with some speed than I am with pure speed. Hence the All Condition Armadillos.