Mostly a rock and glass snail that are all about the algae. They consume algae, cyanobacteria, and diatoms. They don't move very fast but the areas they have been in are clean as a bell. These guys like to crawl outta the water some, but I have never found one that "jumped ship." These are great snails, but also have trouble turning themselves back over.
They mostly stay in the sand, but will sometimes make short trips up the glass. They are mainly detritus eaters and do an excellent job cleaning and shifting the sand. I think these are also some of the most hardy snails out there. These guys can flip themselves upright very easily and quickly.
Super fast grazer that will knock out some algae in no time. They mostly stay on rocks and glass, but will work the top of the sandbed too. They are cold water snails that don't always do so well in a reef. But, once they are adjusted... they seem to be very hardy and active.
Conchs eat diatoms algae that grows on live sand beds. Fighting Conchs will bury themselves in the sand bed and clean the upper layers. These guys are all about the sand bed and are very cool to watch. They usually don't get flipped over, but if they do, they can kick around and turn themselves back upright. They very hardy too.
Based on that...I would add the Nass and the Conchs first. Nass are great for the general clean up and the conchs will target the diatoms that new tanks are prone to.