Generally speaking, a deck's skill ceiling comes from the amount of decisions a player has to make during a game, as well as the difficulty in getting those decisions right. As
@Merovingian pointed out, control decks are usually the best example of these because you're usually selecting and playing multiple cards on any given turn, and messing up the sequencing will spell doom for your attempts at locking your way to victory. In general, however, the more moving parts a deck has, and the longer it takes to complete the deck's objective, the more skill-intensive it's going to be.
Both of the current Tier 1 decks in Standard, Buzzwole/Lycanroc and Malamar, have various decisions that need to be made, of course. While both of them are focusing on spewing a bunch of energy onto the board, the important distinction is in where that energy goes. On paper, Malamar would take more skill because every turn you're trying to accelerate multiple energy and figuring out where to distribute it, but Buzzwole has a lot of little micro-decisions like how Octillery BKT interacts with your draw supporters and whether or not to play a Guzma or similar instead, when to use Lycanroc-GX and when to power it up as opposed to attaching to a Buzzwole, using a Beast Ring to only fetch 1 Energy, using Absorbtion-GX vs. opting to save the GX move for Dangerous Rogue and whether you think the Buzzwole will be return KO'd, etc.
Buzzwole and Malamar are both easier to pilot than something like Sylveon, however, because with Sylveon you're trying to pick 3 cards out every turn and
anticipate the progression of the game state then react to it accordingly, and card sequencing plays a greater role when you take into account disruptive supporters, potential Lusamine Loop establishment, timing of Plea-GX, when to go on the offensive with Fairy Wind... it is a very skill-intensive deck.
Overall I'd say Buzzwole is probably the most straightforward Standard deck to pilot at the moment, but there aren't really any brainless decks out there right now.