like everyone says, it depends on the situation.
maylsian trumpet snails are good for stiring up the sand and helping stop gas pockets forming, also east left overs and dying leaves. they reproduce quickley and hard shells make bad food choice for snail eaters.
ramshorn snails are good as the look colorful (come in brown, red, blur and black) eat dying plant leaves and algea. may eat live plants if not enough food. need a mate to reproduce
apple snails, look decorative, come in many colors and eat leftovers and algea and anything else it its way, they grow hugh and produce a lot of waste. need a matre to reproduce.
pond snails/common snails, easy to look after, usally live on leftovers and plant matter.easy for snail eating fish to eat.breed quickley and can get into filters, making it a pain in but to get rid of.