Oh man, struggling to decide.
Gotta give props to Socom 1 and especially BF2: MC. Socom 1 was my first real multiplayer shooter, and it was a hell of an introduction. With no respawns, it really taught me to be careful with my shooting. It was also just a blast in terms of culture since almost everyone had a microphone (since it was bundled). Socom 1 taught me the basic tactics I used in shooters afterwards.
BF2: MC. It's a real tossup between that and Bad Company 1 for my favorite shooter. But BF2: MC is probably the most important game for me because it introduced me to the Battlefield series. It was almost magical how it happened too. When I went to go pick up the mail, there was a mysterious CD package, I think it was all army green and whatnot, and inside was a demo CD of Battlefield 2: Modern Combat for the PS2. Dunno how EA knew, or how much money it costed them, but that promo got me to buy 4+ Battlefield games ever since.
The demo was the map Backstab looped over and over again, and holy shit, that was an amazing map. You had the MEC side with 2 Helicopters, with everyone on the team jumping in and then parachuting down into the main city in the middle. The Americans had 2 tanks and you just had this real sense of suspense as the two teams converged towards the city in the middle.
And man, were the battles hectic. You had a river in the middle, lots of buildings you could go into, etc. You had trenches where you could jump into (I remember once putting C4 underneath so that when a tank rolled over my head, I blew it up). You had anti-air guns. You had civilian cars.
Best of all, you had these "artillery strikes", which the game would announce being available. The artillery strike was basically a nuke, where giant white streams/beams would fall from the sky and annihilate almost everything that was out in the open (you still had a good chance of dying even if you had something over your head, so you're still tense as you huddle underneath a roof). The strike was controlled by a terminal in a little house to the offskirts of the city, so everyone would just immediately cease their firefights and high tail it to the terminal. Even if you got there first, you never knew if someone else just arrived behind you so there was just a lot of excitement and tension.
And oh man, capture the flag, the huge number and variety of maps. I mean, I can't believe just how many maps there were with the amount of variety and creativity. I'm not exaggerating. There was map with this giant metal gate that blocked the enemy attacking team, which had lots of superior vehicles (which were useless as long as the gate was closed). So pretty much, everything depended on whether or not the gate stayed closed, and there was just a furious battle over that. There was a beautiful map, where the Americans have an air base on a beach, and attack a giant oil rig via helicopter. It's just beautiful flying across the ocean, red from the sunset. You could also attack from the bottom via boats too.
Long story short, Battlefield 2: Modern Combat is why I play Battlefield 4, even though it's fucking terrible. It feels like a lot of love and care was put in the game. Not in terms of technicality, but in terms of the maps and gameplay choices. The designers knew how they wanted the game played, and designed the game so that players play it that way.
Bad Company 1 was a huge blast for me too, just because of the destruction. I can't overstate how game changing destruction was. It too, also felt like the designers knew what the theme of the game was and centered their design choices around it. I also loved it just for the tone. Playing Bad Company in the summer was awesome with its music.
