Don't know about pure OpenCL.
Intel, has over all well support and not too bad compared to windows counterpart drivers. It lacks OpenGL 4 features. It only uses OpenGL3.
AMD has open source drivers, which mostly suck. Performance in games is rather bad. Same as synthetic tests. Many people report about rather good bitcoin performance.
Proprietary drivers are a bit better on performance but worse on stability.
Nvidia, has 2 types of drivers. Reverse engineered ones, which suck and blow when it comes to performance and binary ones. Nvidia with binary drivers has highest performance + stability in games. It's on pair with AMD on bitcoin stuff.
In reality no one can say what is good for OpenCL in Linux, because there are no OpenCL apps in Linux. I am no Nvidia fanboy, but i am Nvidia user for a while. They are safest bet on quality/performance, but AMD may as well worth your while, especially if you are dev and don't mind reporting issue to kernel devs, etc.