I like CUCs, they are fun to plan.
I really like the following...
Banded trochus - I know that if they get flipped upside down, they die, but they do a good job eating algae and they are just so festive to look at, especially if you get a young snail that doesn't have a lot of algae growing on it. the bands are iredescent maroon and pearl. It's a pretty snail.
ceriths - they work very hard and will eat pretty much anything that you want them to eat, without touching anything that you don't want them to eat.
Nassarius snails - get N. vibex. Great snail and very fun to watch as it pops out of the substrate when foods on the table. A sand sifter.
Stomatellas - One of my favorite snails. Excellent cleaners.
Hermit crabs - I like them, though they can be bumbling sometimes and just walk all over everything. I don't like mixing species, but others do it without a problem. I've kept 3 species...
1. Scarlet hermits, they are like on valium. They move slow, they eat, and they are very peaceful. They also don't kill for shells as much as I've seen other hermits do.
2. Blue-legged hermits, they are like on speed. They move fast, they'll eat anything, and they are pretty peaceful too. They change shells like I change my shoes! They've killed a few snails in my 36g and to be honest, if I had to do it all over again, I'd replace these guys with scarlets. They kind of disrupt my seagrass beds a bit. They also walk all over everything. All hermits do, but I don't notice the scarlets causing as much disruption.
3. Electric Blue hermits - These guys are on vodka. Very clumsy, rather big. I love them to death, but they'll walk and knock everything down, and I've got little ones. They can get to like 2". I can trade mine at the lfs when they get too big and get little ones.
Emerald crabs - I like mine, but mine is a little one and he behaves.
I could go on. In MARC there is a series of good threads by Steelhealr about the CUC. For the most part, it's very good. A few more species need to be added though.
L