I tried making Marianne a dancer in the run I just finished. She worked out well, no regrets. Between Sword Avoid +20 and Axebreaker, axe units pretty much couldn't hit her, and she wasn't really scared of mages due to her high res. In theory should have had decent combat with her relic/sword dance/high charm. However, I don't think she ever saw combat, aside from blicking some random armor with her relic on enemy phase once. The riding proficiency is cool, but she never did make it to A+ for that sweet movement +1, so I don't know if I would count that in her favor or not. I guess without the proficiency you are even more unlikely to reach it.
Anyways, after giving the topic much thought I think Annette is probably my vote for best dancer. She seemed really mediocre as a combat unit in my BL run, but had some really nice rallies and a strength in authority. In my experience with the game so far, rallies have been really useful early-game for hitting 1RKO benchmarks quicker to help units snowball. However as you progress into the mid-game they lose a lot of value due to the combination of being locked to a 4 move unit and generally not being needed to 1RKO things. You will also notice that if a unit is using their turns to rally, they are probably not gaining much exp.
So if you are using Annette for rally support, you should end up with an under-leveled unit with lackluster combat, a mediocre spell list, and low move (unless you make her a cav/peg for pure support). What a perfect dancer candidate! Icing on the cake is that focus on authority to get rallies is now going to help you get the required authority for the "dance of the goddess" battalions faster.
The timing works out pretty great too. Once rallybot Annette starts falling off in usefulness, the dance competition is just around the corner to make her viable again. Her combat will still suck as a dancer and she won't make great use of its sword skills, but is she ever actually going to fight anyways? Even if a dancer CAN fight, are you ever actually going to use them to kill an enemy instead of refreshing an ally?