Visually there is no contest. Witcher 3 is alongside GTAV probably the most elaborate ''open world'' in the industry. I have never made a secret about that fact on this forum.
In terms of everything else, both Ass Creed and Witcher 3 is very contained. Ass Creed has at least some free parcours, tower climbing and stealth to reiterate it's ''openworldness''. It's combat is straight forward and could be not more open world than any other simple action game. The sea battles in Ass4 was decent, but pretty simple, boring and repitative after awhile.
Witcher 3 combat is a bit more elaborate than Ass Creed. There are some cool bosses that use the air and ground. But that combat could have easily worked in a straight forward 3d action game, like God of War, Darksiders or whatever. Half assed horse riding and boat riding/combat is fuckking whack. Never felt so restricted (anti open world) ever in a videogame during those mechanics, since the 8bit era.
Uncharted 4 has open area stealth, terrain dynamic, multiple paths to tackle, vertical climbing, rope takedowns/traversing, vehicle action, underwater stealth with some decent physics and collision, all combined in a nice little 'free flowing' sandbox style gameplay.
''True'' open world game in MY definition:
-GTA, Just Cause, Far Cry, Crysis. MGSV.