I would agree with Peachy being the most common but Cotton Candy is pretty close behind.
However, for an army you should really pick a pony you like; not necessarily the pony easiest to come across but a pony you like. Yes, making it a reasonably priced pony is a good idea as Mimic armies could be pretty expensive to gather. But the fun of collecting an army is to do it gradually and enjoy the pony you are collecting.