QM2 is not really a good choice. The Queens Room is often used for lifted-pinky, invite-only closed receptions. Access to the alternate venue G32, is by way of the QR, so it's often not available. On Queen Elizabeth and Queen Victoria, the alternate venue is on Deck 10. Also, in the past few years the QR Orchestras have stuggled to reach mediocre. The Caribbean groups upstairs have been surprisingly good when given an opportunity.
In the past, Costa has been the best choice, decent music and floors and a wide variety of music. It has attracted a loyal following of Europeans, although you'll see many dances more common in Europe, like Paso Doble and Mazurka. At present however, the situation seems pretty confused. (My last cruise was only 25% full, so cutbacks were inevitable.)
I wouldn't recommend any American cruise line. NCL's Manahattan Room would have been a good floor if the restarant people hadn't covered it with tables, which seemed to sum up NCL's attitude towards dancers. As far as Carnival out of San Juan, well, "lively" is one way of putting it. It was the only time I ever felt like sitting with my back to a wall in the disco.