M. Schumacher, Senna and Montoya are among the hardest position defenders I have seen in F1. They literally played it on the limit with other cars.

Trulli, who has been mentioned here, isn't really as harsh defender as those, which is why he has seemingly "made it at times easy to others" over the years as has been pointed out here. But he still has fine moments of blockings to show like Europe '99, Australia '02 (before spin), Bahrain '07, France '08 from the top of my head.