I play natural selection 2. Both very cool-looking FPS games with an often-overlooked layer of strategic depth, absolutely reliant on cooperation/teamwork (tiny bit less for marines). Also, you can fly in both games. However NS2 is a lot more difficult to play initially because there are no tools to learn (like novice matches, CAs) except built-in outdated tutorial videos, and individual fights are paced in milliseconds. You also aren't constantly running around next to each other, so it can be harder to teach people. However, nothing beats the feeling of gleefully sneaking up on an oblivious marine, or the rush of chomping down an observatory just before a beacon finishes, or forming up in the vents for a massed push on nanogrid.
