I'll second Astonishing. Even if you decide to forgo the trades, I'm sure that--minus the variants--you can pick up at least 1-26 or so for $1 an issue or less.
Other opinions:
1) I'd also suggest Ultimate X-Men, though the series is supposedly coming to an end (has this been officially announced, anyone?). I think it's at issue 99 with a natural stopping point coming up. I didn't start getting it until maybe issue 89, so I got lucky with my lcs with a big sale and picked up the first 5 or 6 hard covers so I didn't have to deal with getting the full back-fill and the space that goes with it.
2) I haven't really been enjoying Uncanny X-Men or X-Men Legacy much. Buy a couple of small runs to test it.
3) And try and pick up the Messiah Complex crossover. I enjoyed it.
4) Stay away from X-Men 2099.
5) Then branch out--I really enjoyed X-Factor lately up until I dropped all by subs.
6) Try and get the Old Man Logan arc in Wolverine--obviously it's not specifically an X-Men title, but it's great!