Battlefield 1942, which currently is free @ Origin.
I liked big maps, lots of different vehicles and teamplay. I disliked emptiness present in most of maps and lack of custom map support - you had to manually download map before joining server causing that most of servers didn't play any custom maps and even if they did, they were mostly empty.
I also liked two official addons, Road to Rome and Secret Weapons of WWII - they had updated maps with more content, but just like custom maps, they were rarely played :/
Battlefield 1942 was awesome! Quite innovative for its time. Although it wasn't the first fps to have vehicles, I think it was the first one to have so many of them and so many different types, all of which functioned so spectacularly well. Truly a blast from the past.