I've used HP-Test Center, Manits, and the more simpler Rally based test plan tool among a bunch of others. HPTC can be a beast if you let it get out of control and get too large a test plan, but has nice tools for testing against a certain release version and is a bit water-fally.
Mantis is good for a scrum based setup. But then again it depends on what your testing, size of the testing staff, and how long they (SMs, BMs, and PMs) give you in a testing cycle. In the more Agile shops, testing has to have a quicker turn around and is more feature-targeted so big systems like HPTC, while nicely thorough, tend to become big projects to maintain. The choice has to meet the type, organizational scope, and Agile-ness of testing project you have.