Whilst I'm not sure that contests are my favourite conceptually, they are my favourite in terms of execution.
Contests just felt a lot more fleshed out than the other side activities. When contests reappeared in Sinnoh after debuting in Hoenn, I hoped that they had become a series staple that would continue to be present in the other games, sadly that was not the case. I preferred contests in ORAS than the earlier games TBH. I haven't actually tried pokestar studios yet but from what I know about it, it could be a close second after contests.