I have been to a decent number of theme parks over the years. These are some of the best (at least in my opinion). Some big named parks may not be on this list, but that's because I haven't been to them.
All of these are parks I've been to within the last 15 years or so. My experience is of course going to be influenced by things not related to the parks themselves, but I'll try to be non-biased. For example, Disneyland is not on this list because every time I go there, everything seems to go wrong. Nasty guests, aggressive birds, sunburns, the works.
Epcot gets a lot of flack, but I really enjoy it every time I visit.
Knott's Berry Farm in California is great.
“All of these are parks I've been to within the last 15 years”
Did you really think Disney wouldn't make the list? It's Disney World!
Nickelodeon Universe in the Mall of America in Minnesota is 100% indoors.
Magic Mountain in California is another favorite.
Universal Orlando is one I recommend more than one day for.