Wow, this is like asking which one of your children you like the most!
Probably my most treasured souvenirs are a couple original WDW ticket books that actually have all the ticket types in them (they aren't complete, but at least it's not just A and B tickets). Looking at those brings back a lot of memories of going to WDW as a child. In second place would probably be a couple of Japanese dolls (not adult looking dolls, but child dolls) I bought years ago at Epcot - they are so not Disney to look at, but totally Disney to me. I had a photo of one in my cubicle at work and a co-worker who was part Japanese noticed and knew right away what it was. I guess they are very collectible there (or were at the time).
My most frequently used souvenirs are my Disney jewelry. I have quite a few Disney pins (fine jewelry, not trading pins), bracelets, rings, watches, etc., that all get rotated through on at least a weekly basis. I'm wearing a couple Mickey rings as I'm typing now. I also have a few t-shirts that are particular favorites - particularly my "paging Mr. Morrow" peoplemover shirt.
My oddest souvenir? The cast member parking ticket I received at CR for parking in guest parking (in duplicate) . . . .I have never worked for Disney and was dining at the former Concourse Steakhouse one night after work. I wonder if they every did find a supervisor to notify . . . . .I don't even have Disney stickers or decals on my car, so why they thought it was a cast member vehicle is beyond me (it was not a popular car make/model or color configuration, so I don't think it was a case of mistaken identity).
I've bought TONS of souvenirs over the years, from antiques in the former MK antique store to Mickey post it notes. I haven't had a case of buyer's remorse yet!