The most fun I've ever had with a game has been had with TF2. It does, however, take a day or two of adjusting. The guns often don't seem as responsive as they should be when coming from games like Battlefield or COD. It's a very different style of game. Something that helped me get more into it was turing on numerical damage indicators, so that I knew I was making progress, even on my first day.
Also keep in mind: each class requires a very different style of play, and none of these styles are similar to more realistic twitch-shooters. You can't charge in, shoot people with guns, and expect them to die before you do, simply because you shot first.
So there is an adjustment period. Once you get a hang of the styles, TF2 becomes a game unmatched in amount of sheer fun. If you want to get a hang of the playing styles faster, you can read guides online, or simply watch how other players play. I learned a number of techniques in spectator mode.