There are many suggestions out there and people have different experiences. I'll give you what mine have been and then you can decide for yourself:
1) astrea snails, the backbone of the cleanup crew; if they fall upside, they die if not righted
2) cerith snails, good glass cleaners
3) turbos/trochus snails...excellent cleaners, but, bulldozers, batten down your corals
4) skunk cleaner shrimp...personality to the max
5) emerald (mithrax crabs)....may eat your bubble algae, good detritivores but can be opportunistic when larger
6) nassarius...good sand sifters
7) hermits...I think they are overrated. If you get them, stick with scarlets
Rule of thumb:
1 snail per 1-2 gallons
1 crab per tank if large
Sample crew for a 20 gallon:
-5 astrea
-5 nassarius
-5 cerith
-1-2 trochus or turbos
-1 skunk cleaner
- 1 mithrax
SH