One pocket, by a country mile.
Three cushion may require a wider range of strokes that one must master, but minor inaccuracies in speed control usually go unpunished. Beating the kiss is a special skill required in three cushion, but, in my view, is more about precise shot design than exquisite cue ball control.
Defensively speaking, 14.1 cue ball control is nearly, but not quite, as challenging as one pocket. You have to make sure you don't leave a shot into any pocket, not just one, but the speed control typically required is easer to produce in 14.1. The need for effective use of billiard knowledge is only occasional in 14.1, but very frequent in one pocket. Finally, in one pocket you must odten play defense when there are many loose balls, which is rarely so in 14.1. Finally, one pocket racks have an endgame that require extraordinary cue ball control, while few straight pool racks have an endgame. On the offensive side, of course, there is no comparison. It requires exquisite cue ball control to keep running balls into a single pocket.
Bank pool probably deserves mention, too, as a game in which cue ball control is difficult if one considers all the permutations of beating the kiss, playing shape, and accounting for defense as one plays offense.
Still, the combination of stroke and speed control required to succeed in one pocket is simply mind bending, as anything short of great precision in either cue ball direction or cue ball speed tends to produce a poor result. The shot variety in one pocket greatly exceeds the other games, too.
No comparison here. One pocket gets the nod as the game requiring the best cue ball control.