The price cut would be the big one and also a game that could define Sony as a console. Before it was MGS4, but since that series is no longer exclusive, I say it would be either GOW3, Uncharted 2, or the next Killzone game. GT5 will be a big seller, but people would not buy a console for a racing game; usually people lean towards action/adventure/shooter titles. Sony needs a price drop desperately and also super exclusive content that is on par with MGS4 in terms of popularity.
The only thing that is bothering me is this: Sony could cut the price and their units will sell like crazy and narrow the gap between MS and Sony, but how would that help Sony? Sony might be better off statistically, but the fact is this: they will lose so much money on every console they sell that the hole will only get deeper. Unless they cut down on production costs by releasing a slim, they will not make money and a price cut is not practical.