It's always tough to play "completely" peaceful or "completely' warring. The game is designed for a mixed strategy.
Your question, as you posed it, is likely to bring up a "my favorite UU" thread if it gets too many replies. The Celts seem like a good idea by your suggestion (read a recent GOTM on civfanatics for a good suggestion of what they can do). If you want to wait for a civ who has its UU later in the game, maybe you'd want to play as a civ whose UU replaces the cavalry.
Continents balance the UUs better than other settings. Pangea land mass favors UUs that have increased movement (Celts, Zulu, Aztec, Iroquois), where archipeligo favors UUs with increased attack but not movement (Persia, and especially Scandanavia, with its early marine ability and whopping 6 attack!).
It all depends on your game settings and preferences.