My favorite is the Dreams Tower also, followed closely by the Magic Tower (main lobby tower). I avoid the Wonder Tower (Bonita, as mentioned above). It feels to me like you've been put out back in the MIL unit

It's (relatively) far from everything in the hotel and farthest when walking into the park. The elevators are SLOW, or were on our 2 stays. However, there are some nice waterfalls and koi ponds directly in front which are pretty.
The Magic Tower is very convenient because an elevator ride takes you down to dining, gift shops, guest services, etc. On the pool side, you have lovely views from basically any floor. On the other side, you can see the fireworks though the view itself isn't that great.
The Dreams Tower is my favorite because there isn't a bad view (I don't think) and it's so close to DTD. The walk into the parks is just a straight shot and the monorail is right there. It's easy to get to everything from the Dreams Tower. You can get a standard room in Dreams, but there are fewer because they're only the ones that have a blocked view due to being lower down or maybe on a far corner. If you get a view room in Dreams Tower, be careful of the 7th floor because the palm trees often block your view! We had a corner room on the 7th floor and it was really cool, even though there was a palm tree in the way. WE could see the Mickey Sorcerer's hat right out of our side window (corner rooms have a small side window). We reserved a standard room and requested Dreams but were given a room in Magic. We asked if there was anything in Dreams and were told no. That was ok until we saw the room! It was DINKY!! Only large enough for a double bed! We went back to the desk and requested the room we'd booked (from a different CM). They moved us to a king suite in Dreams
They're used to being asked about upgrades and they know you mean free upgrade.

It never hurts to ask. The worst that can happen is they say no. I don't necessarily believe the desk CMs, though, because I think they often tell you the place is booked to avert requests for change. I've never gone when they haven't told me it was booked. However, I've been able to move twice when my room wasn't ok so I'm guessing it's a standard line to say they're booked.