Welcome to the forum!
Which piranha you want depends on tank size and what you want out of it.
If you want the classic shoaling piranhas, you can’t go wrong with red bellies. Caribes also work, ternetzi are a bit harder to come by. Piraya get much larger so would need a bigger tank.
If you want a solo piranha, any of the serrasalmus are good options. The classic is the rhom, but they are one of the biggest and need a large tank. For a smaller tank, sanchezi can work.