The Valk is probably the most popular right now, but I don't like it. My vote would be the Gans Air, and maybe the Weilong GTS second. There is no "best" cube for 3x3 at this point, just an assortment of "modern" cubes that are all a bit different from each other.
If you don't have very much experience setting cubes up and don't want to take the time, I'd recommend you buy a Cubicle Premium version of whichever cube you decide. It's not worth it to everyone, but if you just want a cube that's good and you don't have to tinker with then those are great.
I am willing to buy another 3x3 if I could know which cubes are the best and used by the best!
Well, for the mains of the fastest 3x3 speedcubers...
Feliks: Gans 356 Air UM
Mats: Valk 3 (He used a Valk M for his WR, but I don't know if that's his main)
Lucas: Either the WeiLong GTS M or the Valk 3, hard to tell.
Sebastian: Gans 356 Air, not sure if he uses the original, U, or UM.
Those are the ones that I know, there are plenty of other fast people that I'm either not thinking of or don't know their mains.