Join Date: Sep 2004
Location: Arguing with the Sun, YT, Canada.
Interest: Hiking, watercolour painting, chasing wildlife, backcountry cooking
I'm going to make an assumption that if it's your first overnight pack then this will be your first big overnighter. If you're young and strong and don't have back/shoulder injuries, either the MEC packs or a used higher-end pack would be a good choice.
Over time your preferences will change and you'll want to upgrade one way or another; you might go more fast and light or more luxury camping which require different packs.
For myself, I bought an Arcteryx Bora 80L pack 15 years ago. It's too heavy for general use now but it still comes out periodically if conditions warrant it.