Although snails get some bad names, mostly when people get hitchhikers or pest snails who reproduce unexpectedly, a vast majority are actually highly beneficial to the ecosystem. Some of these are extremely important if you have any live plants (they can eat dead leaves, clean algae from the leaves, etc.), where others are almost required for those who have a sand substrate (to avoid ammonia gas bubbles that can build up over time).
Which ones do you always try to have in your tank, and what is the reason for having so?