If we're going by stars, it HAS to be Lebron for the past decade. There really isn't anyone else that's close.
You are definitely correct if you include stars. LeBron has been the toughest opponent.
The OP's original post though...
" You know, these players that can are often role players, sometimes even scrubs, but who, for whatever reason, play lights out against the Celtics and thus that you are really worried about when they are on the court ?"
LeBron played lights out against every team in the League almost all the time. If we are including stars, then just about any SA Spur starter would fit the bill.(Since we only played SA twice a year we say LeBron. Playing the Spurs 4 times a year we might feel Tim Duncan would get our vote.