What is best primer to use on oil base paint?

I need to paint over oil base with latex paint and need to know best ways to prep oil base for latex?What is best primer to use on oil base paint?
BEST in the sense of a brand name is probably relative. I use KILZ Oil Base over everything, then latex over it, and it works every time.What is best primer to use on oil base paint?
The old rule of thumb is latex over oil but never oil over latex. sand the oil and paint the latex over it. you will have to prime only if you are changing color or sheen otherwise the existing oil is a primer.
I use KILZ. Make sure you get the oil based.