My favorite is Carousel Inn, but would also consider the following if Carousel was booked: Best Western Park Place, Tropicana, Del Sol, Fairfield Inn and Howard Johsnon. These are all located right across the street from the main entrance on Harbor Blvd.
I've stayed at each of these and have found them all to be clean and provide enough space. We don't spend much time in our room, so all we need is a clean, comfortable place to sleep, which these all provide. If you plan on spending more time in your room or using the pool, then the Howard Johnson is great. It provide more of a resort feel, than just a place to sleep. You actually have a choice of two decent sized pools, while the other places have your average backyard pool.
One drawback of the HoJo is no microwave. Not a necessity, but nice to have to heat up leftovers or pop popcorn. Also, there is no continental breakfast at HoJos. Not that its all that great at the other places, but it's nice to grab some toast and fruit before heading into the park.
The Carousel has an awesome breakfast room up on the 5th floor, overlooking the park. A nice start to the day and you can view the fireworks from here if you didn't make it all day in the park.