My favorite has been Cotto. He was a beast at 140 and landed thunder to the body. He beat a prime Mosley and the best Zab Judah to enter the ring. He fought everybody in front of him and handled himself with class along the way. The most accomplished is obviously Pac.
Hopkins. He was the undisputed champion at middleweight and lineal at light heavy. 10 years of dominance in one weight class warrants him best fighter of the last 10 years.