Turf Takeover is my go-to for Character Leveling because most games run through several of the capture points so the game lasts longer and you have a chance for a decently large number of vanquishes which are worth 200XP each, plus all the supplemental XP you can earn with assists and so on. It also usually provides a decent number of coins depending on your own performance.
Garden and Graveyard Ops are my go to when I want to earn coins more than I want XP because the AI opponents aren't worth much when vanquished so leveling up can take much longer.
Right now I have every character, and my Sticker Book still has roughly 2000 items left to go but I'm not aggressively buying Packs for items and since I'm focused on leveling up I'm skipping the Cats vs Dinos events since they don't contribute to that at all.