I've got a boring but strong combo:
Rebuild + Ferry
Rebuild is a force in unto itself, but Ferry makes it indisputably dominant. There is no need to buy anything other than Rebuild (and possibly Estate) with this strategy. The fastest approach is to not buy any Estates and empty the Province pile ASAP. This takes about 9-10 turns on average. Against certain opposition, it probably makes more sense to buy a couple Estates to lock down over half of the VP. A third card is only slightly useful. Scouting Party probably provides the most reliability. The fastest Province rush I can think of takes 7 turns (ending with three Provinces). This kind of speed requires a lot of good luck:
Rebuild + Ferry - 7 turns
1: CCCCE - Buy Ferry (on Rebuild)
2: CCCEE - Buy Rebuild
3: _RCCCE-CCCCE - Trash Estate, Gain Duchy, Buy Rebuild
4: E_RCCC-CCCCED - Trash Duchy, Gain Province, Buy Rebuild
5: R_RCCC-CCCCEEP-R_CCCCEEP - Trash Province x2, Gain Province x2, Buy Rebuild
6: R_RRRC-E-P-CCCCCCE_D-P - Trash Estate, Duchy, Province x2, Gain Duchy, Province x3
7: _RRRRC-P-E-CCCCCCP_D - Trash Estate, Duchy, Province, Gain Duchy, Province x2
