When it comes to single player shooters
DOOM and Wolfenstein - These games have aged like wine over the years! Doom especially, I first played this when I was 5 years old and it has NEVER ceased to get old! I also love the community that has made all these mods over the years for Doom.
Also for any Doom fans get the Brutal Doom mod, while Doom is already an awesome experience the Brutal Doom mod will enhance it 10 fold! JUST LOOK AT THIS BEAUTY!!
Also get ZDaemon if you want to play Doom on multiplayer. I've actually tried it and it's pretty fun. Problem is none of my friends seem to want to play, it's always modern games with them. Seriously guys, there's nothing wrong with playing older games, you'll always find something that's pretty fun to play.
When it comes to multiplayer shooters
Red Orchestra 2: Heroes of Stalingrad/Rising Storm
This is a game that I love because of its realism and I'm not talking about CoD or Battlefield realism I mean REALISM. There's no recovering health, you either bandage yourself up to stop the bleeding or you will bleed to death and that's only if the would isn't fatal. You actually have to watch where you get hit. Also the hud won't tell you how ammo you have in your magazine, you actually have to check the magazine yourself and it won't give you a specific number, only that it's full, close to full, half full or close to empty. There's no crosshair so you'll have to half to rely on your iron sights or else you'll pretty much be firing blind. The covering system is also handled realistically, you half to peak from your cover to aim and shoot at a target or if you have a SMG you also have the option to blindly fire from cover. It even extends to Machine guns like the Browning 50. Cal and MG-42, if you continuously fire and your barrel overheats it's useless and you'll have to replace it. As for tanks (which is exclusive to the Eastern Front) if you decide to take the position of tank commander it's best that you get other players to form a tank crew or else you'll be shifting positions to get from the driver's seat to a weapon (and it'll show you shifting positions in First Person as well, they went as far as to recreate the entire interior of the tank). I have to say while it's very VERY realistic it has proven to be one of the funnest shooters if not THE funnest shooter I have ever played, especially if you like to think and strategize in your shooters. Whoever likes shooters give this a shot!
Other than that
Battlefield 1942
Battlefield 2 Battlefield Bad Company 2 (Plus Vietnam expansion)
Team Fortress 2
...You know this is kind of hilarious when you think about it. For someone who loves a show about magical talking ponies that go around on adventures and spread a message of friendship, love and tolerance a lot of the movies, comics and games I enjoy are pretty damn violent in comparison. XD