There is no such thing as "without bottleneck", you will always bottleneck on either the CPU or GPU unless you hit the game's refresh rate limit. If you play with vsync on, then the monitor's refresh rate becomes another possible bottleneck.
The real question would be: "What GPU would be the best match for an i5-4590 in games X, Y, Z at resolution XX, YY details, etc. with a frame rate in the XXX-YYY fps range" as what people usually mean to ask when asking for "the most powerful GPU" actually is "what GPU should I bother with?" The answer depends heavily on how little/much you are willing to compromise on graphics. If your expectations are highly malleable, you may be able to get away with a GT1030 or less. If you demand everything at Ultra, even a 1080Ti might not be enough, though the i5-4590 is bound to have some trouble keeping up with it in many games.
The GTX1060 is the middle-ground: not over-powered to the point that most of the GPU's performance ends up sitting idle most of the time unless you go out of your way to inflate the GPU load.