Short version: I think Big Hero 6 is almost as good as Wreck-It Ralph, which puts it about on par with Frozen in my book.
The pros: Baymax being awesomely adorable. The super-heroes being awesome. A lot of focus on familial love but absolutely no romance. (Not that i have a problem with romance in general, it's just a nice change of pace not to have it dominating the story.) A very deliberately multi-racial cast and multi-cultural setting. That setting is really really beautiful. Also, there's flying. (I wouldn't have thought to compare that bit to How to Train Your Dragon myself, but since someone else did, yes, a lot like that.) Oh, and a better than average post-credits stinger.
The cons: A very simple and straight-forward plot. Both Wreck-It Ralph and Frozen had more complications and twists than this. A slightly sub-par soundtrack. It's definitely not a musical, and although i like Fall Out Boy well enough i'm even more partial to Owl City and crazy J-pop music. (Come to think of it, it's a little strange that Wreck-It Ralph had more J-pop than the movie set in San Fransokyo.) And at least half of the really cute Baymax things have already been shown in trailers.
Back to that plot thing. There are two things in the movie that are sort of plot twists. Except one of them has been given away by pretty much all of the trailers. As for the second one, it really seemed more perplexing that surprising. They tried to sell it but i wasn't really 100% convinced.
All in all though it was a wonderful movie. Falling a little short of Wreck-It Ralph is praising with faint damn, and i definitely enjoyed it more than Interstellar, which i shall get into in just a bit...