I went to Universal last year around the holidays & the place was EMPTY! Rides were all walk-on; the only wait we had was about 10 minutes for the studio tour tram & for the Shrek show.
I've been to Knott's around the holidays for the past several years and, although it's definitely not *empty* like Universal was, the wait times are pretty minimal. You must see the Snoopy holiday ice show; it's super-cute and included with park admission. Also, if you'll be there on a Sunday, there's a great brunch buffet at Mrs. Knott's restaurant where you can meet Snoopy and have your picture taken with him.
Can't comment on the other 2 places as I've not been to Sea World in about 25 years & I've never been to Legoland...