I played Tower Assault. I don't think I ever played AB1 or 2. I know one thing: don't bother with the new Alien Breeds which Team 17 released a couple of months ago on Steam. They are utterly boring. The Shadowgrounds games are much, much better and for a modern multiplayer Alien Breed check out the free Alien Swarm.
Team 17 really lost their touch 15 years ago :\.