I'm very curious to see how Pokemon Trainer performs on the competitive scene this time around. It's interesting that many of the first impression "tier lists" I've seen rank the different Pokemon separately. I suppose that helps to give an idea of their overall power levels, but it's important to remember that effectively using all of the available options is the optimal playstyle. As others have mentioned, a fighter that can switch between multiple characters with different roles and archetypes will always be at least a little bit better than the best of those characters. Even if two out of three were dead weight, the fighter as a whole wouldn't be any worse than the lone effective option because the optimal strategy would be to stick to playing that character. As it is, Squirtle, Ivysaur, and Charizard all look like they complement one another well. Time will tell how "strong" each of the Pokemon is individually in terms of matchups, but I have faith that the Trainer's flexibility and versatility is going to be a major selling point.
Charizard picked up a number of nice changes since Smash 4, and even if its own comparative strength hasn't shifted much relative to the rest of the roster it brings great options to its team. It's my favorite of the three Pokemon, but I definitely expect to be using primarily Squirtle or Ivysaur depending on whether I need a combo-oriented speedster or a disjointed zoner for a particular matchup. Zard will see a lot of play when it's time to KO or avoid being KO'd, due to its weight, recovery, and powerful kill options. I already miss Rock Smash, but being able to switch to Squirtle with the speed of a spot dodge is a much better way to get out of danger or reset the neutral. I do like how the order of switching allows you to get the Pokemon you most need right away in certain tight spots like recovering (Ivysaur to Charizard) or escaping (Charizard to Squirtle).