Witcher 1 and 2 weren't open world so hard to compare but both were excellent. I haven't played either W3 or BB yet . I know I'll love Bloodborne when I get around to it as I love the Souls games. I also know Witcher 3 will be amazing because I love open world.
Only question for me will be if Witcher 3 can knock Fallout 3 of it's perch as my favorite open world of all time. Something about picking a far off waypoint on my map, turning on the in game radio and listening to music from the 40's and exploring a post apocalyptic world that was amazing! One of those games where the hours just disappeared.