Comparing Hamilton today to Fangio in 1955 makes about as much sense as comparing Hamilton and Rossi. Modern F1 is so different to Fangio's day that no sensible comparison can be made.
All you can do is carry out a thought experiment and try to imagine each in the other's cars. Would Hamilton (or Alonso, Vettel etc) be as quick as Fangio in a Maserati 250F? Almost certainly they would. Would Fangio be as quick in a MP4-25? Of course. Would he be able to drive one for 90 minutes? No way - he'd need to train up to the physical side but his inherent skill would still be relevant.
One thing for sure though - Schumacher wouldn't have tried his infamous crash-to-win tactics in 1955. Not if he'd wanted to live to see his next birthday anyway...