That's easy, any rod that has a fish attached to it.
Actually, it depends on how I'm fishing:
My favorite sunfish/trout/small water rod is the TFO Finesse 8'9" 3wt. I fish this rod a lot.
If I'm going to be casting into the wind or will need to make long casts, I like the TFO Axiom. This rod throws the tightest loops of any rod I've cast.
An 8 wt TFO BVK rod with a TFO BVK reel became my favorite Lake Texoma rod the first time I fished it. The BVK rods have a comfortable, easy to cast action and the light weight of the rod and reel combination is easy to cast all day long. The BVK rods and reels are really good general purpose setups.
To answer your original question, I think you'll find that 2's, 3's, and 4's are the most popular sunfish rods. 2 wt's work well in the small urban streams that many folks on this board fish. I think a 4 wt works real well on ponds and from a kayak; it will handle larger flies and more distance that a 2 wt. Personally, I've found the 3 wt to be a good solid workhorse size for most of the places I fish. But if this will be your first fly rod, I think you'd find either a 4 wt or 5 wt will easier to learn to cast and fish.
Note: the smaller the line wt, equals the smaller the largest fly that the rod will easily cast, and the more challenging it will be to cast when the wind is blowing. Just a couple of things to keep in mind if this is your first fly rod.
Good luck shopping for a rod!