Is it for target archery or for hunting?
If for hunting, then find a favorite broadhead, and get a fieldpoint of the same weight - I like heavy broadheads, but that's personal preference. Mine usually are in the 160 grain range. The only rule here is practice with what you're going to hunt with.
If this is for target archery, then it depends on what ranges you're going to be shooting. I like to go heavy when I'm closer to the target, (160 grain) but I use 125 grain for the longer 100 yard shots. The wind tends to get them a little more, but you don't have to elevate as much.
But, once again, this is personal preference.