The most popular is a variable magnification of around 10-50. As a general rule I think it's fair to say that most FT'ers shoot on anything between 20-35 mag and range find on 50. There are also some fixed mag scopes that are more than useable on the FT circuit; it's all down to personal pref.
As mentioned below, get down to your local club and try out a few of the many options out there.
RAW TM1000 [.177] || S&B PMII 12-50x56 (That's right... just one rifle!)