Woah, you're off to a great start! In general, high gravity beers age really well so I would consider barleywines to be a good bet. I recently had an Alaskan Barleywine from 2007 and it was fantastic. A word of warning, though, I started aging beers several years ago, and it quickly became an obsession and I'm worried that the crew from "Hoarders: Buried Alive" will show up any day now. Decide in the beginning how much space you want to devote to this and then once you fill it up, enact a strict "one in - one out" policy.
Other beers that are worth setting aside:
St Bernardus 12
Dupont Avec Les Bons Voeux
Anything from Cantillon and 3 Fonteinen
I really like it when the beers have dates on them, but if they don't, then you should put a note on them yourself. Oh, and I don't recommend any IPAs for aging - the hops usually turn into a hot mess.