GTA V is probably the best, but I think it is a legit argument to say Red Dead is the best game Rockstar has ever made. Depending on my mood I think it is better than GTA V. The score for Red Dead was amazing. Music in GTA V sucks. There aren't enough things to tweak in customization (only thing you can change are breaks and acceleration). There is no gambling in GTA V even thought they have a casino. Cars often disappear even when you purchase them (big let down). I've walked away from cars in GTA and they aren't there when I come back. Rockstar said they would still be there.
GTA V is one of the best games ever made. It beats GTA IV quite easily but it's not a perfect game. I don't blame Rockstar either, this PS3 and Xbox 360 are old pieces of hardware. They did the best they could given the technology.