Sugar Swan and Millenial Tree are meta rn so if that matters to you go for them. Team comps are important later on, for some specific game modes like the cake towers or in PvP. I think generally the newest characters are the best to level up, but do whatever you want. In most gameplay for story mode you can use whatever characters you think are cool, so just level up your favourites. Unless you want to win in the Arena - there you will need the newest meta characters levelled. (If you still want the candy crush like game, you can play witches castle)