Trochus, cerith, nassarius and nerite are my favorites. how many you need depends on size of your tank. Most of the reef cleaner packages over-sell you on how many snails per gallon they recommend in my opinion.
Astreas and turbos will make short work of the algae on the rocks. Ceriths and nerites will tend to get in-between cracks a little easier. Unfortunately, speaking from experience, snails will sooner leave your rocks sparkling white than clean the entire span of the glass.
You're more likely to see little snail mouth/footprints on the front side of the tank. You could get a magnetic algae scraper and do a once-over everyday to get the loose algae off the glass and into the water column. As a bonus to this method, the suspended algae becomes food for your corals and other filter-feeders!
Also, the nassarius is an awesome snail. They feed on detritus more than algae, but are fun to watch. The snail lies in the sand bed for most of the day, but will POUNCE out onto the surface when the scent of fresh food lingers by! I used to feed more often per day just to watch them scurry around the tank. FUN!