I have all 3 now and am currently playing through the first (reading the novel as well which is nice for some back story and fleshing out the lore) and although not as pretty as 2 or 3 it's still a great game. I really like the story and the enemies are interesting (even though they remind me of the Strogg from Quake
) with some great AI that have flanked me many a time.
Played a bit of 2 and 3 just to see what they're like and it's weird that 2 seems to look prettier than 3, but could be my eyes.