I won't give up any of them! But my Top 10 Favorite MMW are:

1) Amazing Spider-Man Vol. 1
2) Amazing Spider-Man Vol. 2
3) Amazing Spider-Man Vol. 3
4) Amazing Spider-Man Vol. 4
5) Amazing Spider-Man Vol. 5
6) Doctor Strange Vol. 1
7) X-Men Vol. 1
8) X-Men Vol. 2
9) Captain America Vol. 1 (Silver Age, natch)
10) Avengers Vol. 1

Guess you could say the list is Spidey-dominated, but he is my favorite Marvel character and the reason I ever started collecting MMWs back in the day.

I also have a fondness for Thor V.2, Daredevil V.2, and Hulk V.2 (such nostalgia for these, as I had the old Hulk Fireside TPB back in '78!), but I went with Avengers V.1 because it's the first volume of the set.