Both. We love DCL and all things being equal, love to cruise with them... but...
We love other lines as well. We did a Virgin Voyages cruise this past November and it was amazing. The food is incredible (no MDR, all "specialty" restaurants and they are all included) and that hammock on the balcony is heavenly! Outside of that, I think that Princess is our favorite, as we don't feel the need to do anything outside of what you've already paid for in the cruise fare. Also, the Alfredo's pizza is amazing...
We actually had a great cruise on our one Carnival sailing (Sunshine) a few years back and are trying the Mardi Gras in a month or so. We went into that first cruise with really low expectations (more on that in a minute) but had a really great time. Carnival is what it is, but they do have fun. Also, the "comfort food", like Guy's Burger, is infinitely better than say RCCL.
So, RCCL. If you just look through all of the forums, this is the one that everyone seems to hype up, but I don't get all of the hype. We've done Serenade OTS, and more recently, Oasis OTS. We did love the Aqua Show on Oasis OTS, but there are just a bunch of things that just don't make me jump to book another RCCL. The included food options, even on Oasis, are limited. On either ship, there was not a poolside grill option, for instance, and the soft serve ice cream ended at 5:30. There was an included Tex-Mex style place (like Blue Iguana on Carnival), which was good, but to get a burger, etc, you had to go to the buffet. Now there WERE a lot of food places, like a BBQ place, a Sports Bar, a Candy Store, a Johnny Rockets, etc, but they are all additional charge. Just not my thing. Also, don't know why, but it bugs me to have to "check out" towels. Don't have to do that on DCL, Virgin, or Princess... And don't get me started on "Cats"...
Would I go on another RCCL cruise? Sure, but I'd put Virgin and Princess above it, but that is just us. If you are looking for the waterslides and rock walls and all that, then I can see where people like that. As to Carnival, I will be interested to see how the Mardi Gras stacks up, especially being on it so soon after cruising on the Oasis...