So I'm genuinely excited for Dynamax. I think it's got a lot in terms of making it more balanced and strategic.
50-100% HP boost is going to really vary in effectiveness from one Pokemon to the other. Obviously the more HP a mon has the more beneficial it is but on average it lets a pokemon take 1-2 more hits than it normally would. The lack of recovery outside of terrain, seed, and items while maxed also helps control it some in singles while in doubles it becomes a game of partner support. The pokemon also remains vulnerable to percentage based HP loss.
Moves meanwhile are poweful, sure, but nowhere near the power of z-moves. Their secondary effects are amazing of course but I do like some of the balance we see even here. For instance, the strongest a Max Knuckle or Ooze can be is 100 power, which makes a lot of sense considering they boost Atk and SpA respecively. The moves are also easier to absorb because of the lower power and many of the "best effects" have pokemon with immunities to them.
Tying all this together is the two big limitations. Max Pokemon last only 3 turns, and the form ends if they switch out. If you put a pokemon with an unfavorable matchup in, and max them, then you've wasted it. As a quick example, a mixed attacker that was going to run Max Ooze and Knuckle now can't use either if the opponent brings out Aegislash, meaning using other moves to get rid of it, or saccing their max so they can switch something else in. This means that likely Dynamaxing is going to be primarily used late game after the user's checks have been taken out to allow for the sweep. Not only would this let a pokemon do what it was already going to do, just better, but it will make for exciting moments when you finally bring it out.
All that said, I haven't mentioned Gigantamax yet and that's because... well a lot of them look cool but surprisingly I think most of them are inferior to the dynamax options. The most obvious examples to me are Charizard and Coalossal who setup 4 turns of residual damage instead of creating sun or sand respectively. A Charizard with its HA will absolutely prefer sun while Coalossal (bad as it is unfortunately) would prefer sand's SpD boost. Machamp cries at getting a cool form that gets rid of one of the best max moves and replaces it with... minor focus energy. Then there's situational ones. Most cases Corviknight prefers speed boosting over defog and Drednaw would rather set rain than rocks (especially since it gets Swift Swim). There are a few exceptions, of course. Snorlax definitely wants that berry refresh more than dropping opponent speed, and Gengar getting a new way to trap pokemon makes up for losing its mega and is better for it than defense dropping.
My post is getting long so I'll finish it off with this. I think the pokemon that will benefit most from dynamaxing will be bulky offensive pokemon. Stuff like Kommo-o, Hydreigon, and Tyranitar feel like obvious picks especially since they all have methods of setting up before maxing but I'm eager to see what other pokemon are able to do with this.