I agree with Alison, while Legoland is a lot of fun, there is a height requirement for almost all of the rides. We went there when our youngest DD was almost 2 and she only went on two rides total. It was really frustrating for all of us. It was crazy because some of the rides were so slow & tame and I thought for sure my 2yo would be able to ride, but they were pretty strict about their height requirements. I even talked to someone that worked there and they said that there were only 3 rides in all of Legoland that DD could go on.
I've never been to Sea World so I can't give advice about that place. It's next on our "to do" list.
As far as the zoo goes, I think it really depends on your DD. If she's anything like my DDs you may be a little disappointed. Don't get me wrong, I know many kids who love the zoo, but it just doesn't keep the interest of my girls yet. DD#1 (who has always been a HUGE animal lover) didn't seem to enjoy the zoo until she turned 4, and DD#2 is still too young to really enjoy the zoo. My girls seem more interested in the snacks I brought for them than the animals. I think it really depends on the child. Although they do love the play area there!
I think I'm gonna go against the grain on Knott's. First of all let me say that it IN NO WAY compares to DL. It's not even close to a DL experience, so if that's what you're looking for you'll be disappointed. Our family would prefer DL any day hands down! With that said, we have found that our girls really enjoy Camp Snoopy. Unlike Legoland, our youngest DD is able to go on practically all of the rides at Camp Snoopy. They have a little train ride, a mini ferris wheel, a school bus ride, a bounce house, and many more that cater to young children. Whenever we go to Knotts we spend all day in Camp Snoopy and our girls love it. Plus there's Mrs. Knott's Chicken Dinner restaurant that you MUST try if you've never been. Yummy!

Now, I need to add that I don't think Knott's is worth the full admission price. We only go when they have a special promotion or when we haven't had to pay full price. That would be another story since it's not worth it if that's the case.
Have fun wherever you decide to go!!!