Diversity is definitely the best. I use some of basically all of the above. Here are my favorites and the strengths and weaknesses of some.
Sand sifting:
Sand Sifting starfish: Seems to work well, but has his favorite areas he works constantly and doesn't really get to other areas of the reef.
Nassarius: Work great. Seems to diminish over time.
Fighting Conch: Don't know how much they add.
Golden Head Sleeper Goby: Best purchase I ever made. Keeps sand bed white and is a ton of fun to watch. One drawback he will get sand on anything low in the tank and can even bury small items on the sandbed.
Rocks and Glass:
Nerites: My favorite. Seem to eat algae well and do not knock thing over.
Ceriths: Seems to do well both at sand sifting and rock/glass cleaning. However, they tend to be popular shells for the hermit crabs.
Turbo snails: Nothing eats the algae better or bulldozes corals more. A catch-22.
Tuxedo Urchin: Eats a lot of algae. Negative: May eat coraline algae too.
Blue Linkia Starfish: Looks cool. Do not know what he eats, but has been grazing the rocks of my tank for 2 years now for something.
Top Crown snails: Pretty red/white shells, eat great. Get knocked over and eaten by crabs easily.
Hermits: Scarlets seem to be good, but expensive. Blue claws are cheap, but tend to be predators as they get larger.
Emerald Crabs: Wonderful algae eaters. No drawbacks I have found.