Which BW developer is the best for a beginner?

Hands down Kodak D76. Then use it one-shot with 1+1 dilution. That means that after you've mixed your solution (this is called the stock), when you develop a roll you dilute it 1:1 with more water. When this is done, the shelf life of the solution is drastically reduced, so you use it only once, then you discard it (don't pour chems down the drain!)

Why D76 you might ask? Because it's forgiving, it's cheap, it lasts a long time and you can find development times for pretty much every type of film out there easily. If you're new to this, don't make things harder for yourself. The difference between film developers is not big enough for you to care that much about the first year or two. Focus on the photography at this point instead.