For the mini-games, it depends on who your villagers are. You don't need any Amiibo for Puzzle League, and you should be able to do fine on Desert Island Escape without any Amiibo if you have a good variety of villager species in your town. For RVs, it depends on what items you want most.
Isabelle and Mabel are great too. Isabelle for items and Mabel for helping with puzzle league. I really suck at that game but with her special ability I've got much further than I ever would have
This is based entirely on personal opinion, but I found Celeste and Kapp'n to be the ones I like most. I like them as characters, their amiibo's look nice and they give nice items. I was lucky enough to get both as a gift for a national holiday.
Tom Nook is consistently getting me higher scores than Reese. Reese has a longer waiting period (30 clears vs 20 or 25, can't remember which) and always changes the most common fruit (good) to match the least common (awful). Tom changes a random fruit to another random fruit, so there's a decent chance of hitting it big.
Then again, I'm really terrible at the game, so maybe there are strategies I don't know about.