Rings would probably be a good call.
Be sure to balance modulation and utilities (both playable and flexible) with the effects so you're getting the most out of each module.
-- Zacksname
Seconding Rings. It's incredibly easy to make it sound beautiful.
As for effects, I've been enjoying the Nautilus by Qu-Bit. A wide range of very cool clockable watery delay sounds, though it can get noisy a bit too easily. Nice for ambience. With the Rainmaker, however, I suspect you have the delay end of things covered.
Also, bumping the advice about modulation and utilities, this can't be overstated.
Good luck with your rack!