For me it is the likes of Michael Bevan, Steve Waugh and Brian Lara (WI-Aus 98/99 Test Series as one example) of what I have seen as the best players under pressure. Players can also include Brett Lee, Glenn McGrath and Shane Warne. I'll chuck in Chris Cairns as well, he has played some strong innings under pressure.
As for some the worse players under pressure, Lance Klusenser can be included if you remember that Semi-Final, yet he has played well under pressure as well. I guess he belongs in between.