Search the forums using the search function (go back more than a week) . this question, and the topic is discussed an awful lot, and you'll likely have poor response on such a general question.

1) see what other people are using (use the search)
2) check out some of the options
3) Consider all your other gear.
4) try some on and pick something that fits.

Picking a pack and carrying extra weight all year because you "might" do a winter trip is silly. Get something sized right for 3 season, and if you go out in winter seriously, get a big
pack for that, rather than carrying extra stuff all year.

Also helps to fill in your profile at least a little so people know where you are and what you
might be hiking - doesn't need to be everything. Click on my name and you'll get an idea for what I do.