I've done a century (my first) with a stock Tern Link P9. It had the stock Biologic comfort grips on it. They are very similar to the Topeak Ergon grips others are talking about. One thing I learned doing the century, which was quite unexpected, was that the grip surface was just barely wide enough to comfortably rest my elbows on. This gave me a second "hand" position as well as a good tuck position. I think I've gone about as fast as 30 mph that way, so it is surprisingly stable. Due to the seemingly constant rain (outside of the 6 weeks of summer) where I live, locking grips are a must.
Bottom line: flat bar = ergonomic grips (Biologic, Ergon, or BG Contour, all recommended here).