I really enjoyed the Ultra Sport 60. There are a lot of different options: trike gear or tail dragger, retracts, flaps, dual aileron servos or one. It can handle a two or four stroke. I did mine with fully sheeted wings.
The instructions are good. And it is very smooth flying plane.
The build thread started by Minnflyer is as good as the manual.
I have a proctor sopwith camel kit in the basement that will be my long term project. The manual for that one is about three pages. So I guess that is one major difference between some of the 'builder kits' and the rest of them.
I agree with the posts suggesting the sig 4 star and something extra as far as great flying planes but they are not challenging to build unless you do some kit bashing. By the way I fly my 4 star all the time.
If you want to go a little further you could do a scratch build.
I've always thought of this as two different hobbies. The building side and the flying side. It is absolutely wonderful to spend quite time meticulously building a plane. And is just as beautiful to see it break ground for the first time. Where else do two hobbies come together like this?