Ok, I only buy softcovers so here's my top 10 of those:

1 Uncanny X-Men Vol 5
2 Uncanny X-Men Vol 1
3 Uncanny X-Men Vol 2
4 Uncanny X-Men Vol 3
5 Uncanny X-Men Vol 4
6 The X-Men Vol 1
7 The Avengers Vol 1
8 Sub-Mariner (Silver) Vol 1
9 Sub-Mariner (Gold) Vol 1
10 Fantastic Four Vol 1