Self-explanatory. Now that the game is out worldwide, it's a good time to ask. If you're not far enough to have experienced these features, don't worry, I'll be leaving this poll up for a little while.
Anyway, from my experience, it's hard to choose because there's so much amazing stuff. Join Avenue allows you to buy stuff you couldn't ever dream of buying before, and the discounts on level up really provide incentive to keep coming back. Pokéstar Studios may be the best mini-game ever. Pokéthlon was great, but this is like puzzles, except with Pokémon battles. Of course a crit can mean the recording is an insta-fail, but otherwise it's a really clever concept, along with the strange ending mechanic. The Fanservice Tournament may be pure fanservice and a re-skin of Emerald's Battle Dome, but hey, it's really fun with all the modes you can choose from, and all the old characters thrown in for the gratuitous fangirl squeals.
The Hidden Grottoes are a great addition, allowing you to get extra items and DW Pokémon, even though the idea is fairly simple. The Funfest missions are a slight step up from the missions in the first game, but they're still not all that amazing. The medals... well, Pokémon finally hops on the achievement bandwagon, and it works great. I haven't tried the Black Skyscraper / White Treehollow yet, so I can't say anything about it. Same for Easy and Challenge modes, though these are fairly self-explanatory anyway. Same for the Memory Link events. Putting them in the poll anyway.