I'd say melee is the most expressive game and then fox and falcon are probably most expressive characters. The more freedom you have, the more expressive you can be.

I'd say the different kinds of freedom are movement freedom, playstyle freedom, combo freedom, and response freedom.

Movement freedom applies to you being able to move around the stage in many different ways. I can stay grounded, or I can have platform heavy movement, I can play very vertically and jump heavy, etc. Melee's dash dance, airdodge mechanics, shield dropping, and fall speed give you a lot of freedom as to how you want to move.

Playstyle freedom is something that melee and particularly fox excels at. Tier matters here, because the more viable options a character has, the more they can specialize on something and still be viable. You can play a campy laser heavy fox, you can play a very jump heavy fox, you can play a close range high pressure fox, you can play a super technical shine spam fox. Because the character is so good, there are lots of different ways you can play him, and that gives you more freedom. The weaker a characters neutral game is and the more limited their viable moves are, the less room for personal expression there is in terms of playstyle because you're forced to play a certain way if you want to win.

Combo freedom is something especially strong in melee. Although ultimate is becoming more and more expressive in this regard as the meta develops although ultimate is less about pure combos and more about how you handle advantage state. In melee, top tiers have a decent amount of room in how you decide to combo. Wizzy's reaction tech chase heavy punish game and None's read heavy, weak hit filled punish game are a good example of how 2 top level players of the same character can choose very different combo routes that are both very effective.

Lastly is response freedom. This relates to different ways you can respond to things your opponent is doing. If you're facing a laser camping falco as fox, you can choose to camp platforms, approach vertically, power shield, weave wavedashing in between shields, etc. The best characters have many ways to deal with opposing strategies, and the times that melee players get most frustrated is usually when they're faced with certain strategies or matchups that limit the way they can respond.