Rumble roller. It will hurt, but that's kind of the point. Make sure you get the hard one.
Rumble is fine as a general roller, the protrusions actually make it more versatile.
It's all relative.
The actual answer is: Strong and with good enough activation and control to balance out the other muscles inserting into the knee.
A lot of people have a significant deficit in their hamstring strength (poor movement, poor posture), and really all you need to worry about is getting them stronger rather than trying to lift a certain amount of weight (which isn't exactly the same thing as strength...).
A decent barometer for good hamstrings is your ability to do GHRs.