Well, to be a big match it really has to have a mainstream appeal, so something involving Oscar De La Hoya is the most obvious.

ODLH vs Floyd Mayweather would draw in regular fans in De La Hoya, and the approval of the boxing fans as it pits the currebt best in PBF against him. If that isn't possible, ODLH vs Ricky Hatton, who's in the top 10 best and has a big British following, would be big, or Winky Wright perhaps. Hatton is probably too small in stature, as he's too small for welterweight let allown light middleweight, but Winky would be a good match, rougly the same size and respected by hardcore boxing fans.
Other Possibles, due to there size, for ODLH would be guys like Margarito, or perhaps retreads like Mosely, Vargas or Trinidad would most likely attract attention and money.

It's a shame the cold war isn't still going, as Wladimir Klitchko vs Hasim Rachman matches 2 world heavyweight title holders, and the biggest names from America and the former USSR in that division, and with a bit of good communist/capitalist tension going that could be huge. As it is it would still probably be the biggest Heavyweight bout possible, but not the biggest in boxing.

I'm not the most knolegdeable on forein boxing matters, but don't Manny Paquiao and Marco Antonio Barrera have big followings in their countrys? Pitting Peurto Rico vs Mexico worked nicely for Trinidad vs De La Hoya, maybe they could do Mexico vs The Phillipines and drum that up to make that a big rematch.