I started with ME1 only a number of months ago, then tore through 1, 2 and 3 (with a limited amount of cheap DLCs) in quick succession, though technically I still haven't entirely finished 3 (my last save game is somewhere in the final sequence of battles - got distracted and haven't picked it up again, maybe because I'd overdosed a little in the weeks before that). All three are lots of fun in different ways. It's also kind of fascinating if you're a Dragon Age fan how the ME and DA games mirror each other in many ways (and I don't just mean the unsubtle references like the DA logo appearing on Mass Effect armor). Especially in the sense that both series changed in very similar ways from the first to the second game (and the third DA game coming out next month looks like it'll have strong resemblances to the core concept of ME3, as well).
Have to say I'm pleasantly surprised that my laptop - not that old, but a laptop without a real video card, just Intel chips - could handle the whole trilogy without real trouble, albeit long loading times on 3.
Tricky. In many ways I want to say ME1, but after a couple of physics-defying rides with the tank on planet surfaces, it gets pretty old to have to ride around so much (though I understand that on a second playthrough you get more XP, so you could probably afford to ignore more things and only really go for the main mission on each planet). And there's something to be said for the tighter, less cluttered gear systems of the later games. Anyway, I'm always most interested in the story and the characters, which are strong throughout the trilogy - the development in 2 and 3 for some characters are only possible because the foundations were laid in game 1.
Quarians. The asari are cool but a bit cliché perhaps.
Tali, without a doubt. Liara, Garrus and Mordin also. And Joker, though he's not a squad member.
Only done one playthrough so far, with Liara.
For some reason I have a long habit of playing predominantly female characters in RPGs, and this was no exception... on a future playthrough I'll try a male one though.