In RPGs, do you prefer a rigid class system like old WoW, a free-form skill system like Skyrim, or a semi-structured system like Dark Souls?

I'm not a huge fan of the super-ridged WoW system. It works in a competitive game, as it means that everyone of the same class has the same skills, so it is the player's skill in using those abilities maters more. It's also predictable what a class can do, so it is easier to form ad-hoc groups. But it also leads to every character of the same class being essentially the same.

Skyrim's free-form system is nice, but it requires a lot of the game to either have very generic options, or programming in multiple solutions to a problem. Otherwise you could have some player skill builds being useless and unable to complete an objective. See Deus Ex: Revolution as a strong example, where any player who spec'd into non-lethal stealth had serious issues when boss fights started as the bosses were programmed to be immune to pretty much all stealth and non-lethal abilities.

Semi-structured system have the advantages and disadvantages of both systems, though maybe not to the extremes of either system.

Prefer Skyrim's set up 100%

Free-form like Skyrim.

I don't like Skyrim that much, but I love how leveling works it makes sense

Something between the free-form and the semi-structured. The Paths of Exile skill tree system is interesting.