I'd probably have to say TR2. It's just more memorable for me. Between my dad and I we can both say a single line from any of the cutscenes and follow on from it, terrible Italian and Asian accents included.
I play 1 - 5 at least once every year and take TR1 as the refresher. It's simple after finishing it so many times but it has to be done.
2 ramps it up a bit, especially in Barkhang Monastery and the Ice Palace. Venice Violins, the speedboat mine sequence, the snowmobiles and a good old-fashioned dragon. Complete with Mafioso and potentially murderous monks.
3 is, I'd have to agree, the best for environment variety and it has some great accompanying music & ambient soundtracks. It also added some nice new mechanics and more vehicles.
4 went a bit too far with backtracking I feel though. The levels are nice and the rope swinging mechanic, but I feel like I'm dipping deeper into insanity for every time I have to go back through The Great Hypostyle hall and The Nile. And those flame-ghosts can go fuck themselves.
5 is a sadistic son of a *****. Outright the toughest, most dickish entry of the lot. Where to begin and where to end? Those demon babies that throw rocks at you... The guys towards the end that only take hits through that hole in their helmets... Lasers and gas everywhere. This is what the first 4 sets you up for and it cares not for your spare time.
I played a bit of whatever the 6th was but after Chronicles it just seemed like the difficulty dropped off of a cliff, so I ended up sticking to the first 5. I did pick up the pack when it went on sale recently, reboot included, but I've yet to give the newest entry a go. If there's as little actual tomb raiding in it as I'm led to believe, though, I'll be disappointed but not surprised.