Rule of thumb: your Dancer should be either your shittiest unit (lagging behind hard in stats and hardcore underperforming) or someone on the bench.

Yes there are secondary considerations like having a good spell list, affinity with swords, high Charm, but the main consideration is opportunity cost. For instance, Ferdinand theoretically makes a good Dancer, since he can use Swords, Sword Avo +20 synergizes with his personal skill, and he has high charm for Sword Dance, but doing so means you lose out on his incredible usefulness as a Wyvern Lord or a lance user (Swift Strikes).

This is why Flayn is such a popular dancer. She's not a particularly great unit to start with and by the time you get her, you already have dedicated healers and mages and she has no place on your team. Thus, there is no cost to making her a dancer since you get her for free and you were never going to use her anyway.

If you're on BE, then Flayn's obviously not an option, in which case just pick someone off the bench. Recruit a random character you wouldn't otherwise use and make them your dancer. Marianne makes a particularly good Dancer imo.