Schneider, Rodenstock are generally regarded as the best lenses for enlargers, the Nikkor ones are good but the Rondenstock 50mm I have is a much nicer design with a light pipe in the lens assembly to show what f-stop you currently have. A nice little touch that makes life much easier.

As for sizing, m39 is the usual fitting but as said above there are other sizes but you can get plates to convert a smaller lens to fit your enlarger.

Each film format has its on focal length that is to be used.
35mm film uses a 50mm lens
6x45 uses a 75mm
6x6 uses 80mm
6x7 uses either a 90mm or 100mm
As a guide look at what is considered a 'normal' lens for the format you are shooting and use an enlarger lens of that size.
Hope that helps.