Actually I'm not really one to buy theme-park souvenirs, apart from the edible kind. :) As far as funny hats go, which I can be extremely attracted to, my suggestion would be for variety, both themed and unthemed. For example, the past couple seasons at Disneyland I've been somewhat disenchanted at a lack of variety, especially when I see some that other guests have (designs that are out of production), or ones on ebay from overseas DL resorts.
So far, the best souvenirs I've gotten came from the former Marine World (LONG before Sick Flags warped them into the abomination they are today), and from the Mexican parks. The old Marine World yielded several items, including old-logo (the "Elefish") and new-logo jackets, a polo shirt, and some mugs.
Xcaret and Xel Ha both yielded gorgeous charcoal-gray polo shirts, with an embroidered Via Delphi logo (this is the same one that some of the trainers wear when off-duty) and the park name just above it. I also found a hand-painted mug, done in cobalt-blue glass. That one, although practical, doesn't get used. It's truly a show piece.
My wife is nearly as picky, but she was a Disneyland regular in her childhood. She's got a couple of large-size 'Tigger' mugs (they're nearly big enough to qualify as steins, I think). They're ceramic, done in dark-green glaze with raised reliefs of Tigger around the outside. One is in use, the other is stashed away as another show piece.
Beach towels are another good score, if you can find such that have decent artwork.
Keep the peace(es).
One of my all-time favorites is an Animal Kingdom shirt with Winnie the Pooh looking through a magnifying glass at a bengal tiger saying "Tigger, is that you?" Priceless.
Many parks at least have some kind of souvenir photo book, like Disneyland has always had. But sometimes there are even better books, that give the whole history of the park, and I can never resist those.
It's easy to get books about the Disney parks, but it can be harder to find books about other parks. One of my favorites is a book on Blackpool Pleasure Beach, and probably my rarest book is on Luna Park, a defunct park in Seattle.