There is no best snail IMO, they all have different tastes although many of them have overlaping tastes and will eat more than one type of algae. Better to go with diversity, and get as many different types as your tank will support.
2 turbos is alot for a 12g, the 4 astrea is a good amount, 2 or 3 ceriths would be good, a few nassarius ( these will eat left over fish food ), collumbellid grazers stay small and will reproduce in your tank.