Well hang on, we really need to consider all possible avenues for where he could possibly go. In descending order of likeliness:
Man City: Yes, they're big rivals and yes, he'd get a jumbo-sized amount of stick from United fans, but money talks big time. An £80 mil deal with City (small change to them) would give United a big wodge of cash to spend, or more likely to allow to be nicked by the Glazers. However, one has to consider where Rooney would fit into a team top-heavy with strikers: yes, he's better than all of them, but would Tevez be happy dropping back into a wide, supporting role? If he did, where would Johnson, Milner, Adebayor et al fit in?
Real Madrid: Financially impressive, and a team abroad to keep Rooney away from the British press. However, abroad is both a boon and a burden. Quite apart from learning a new style of play, culture ang language, British players have historically never done too well (honourable mentions such as Lineker aside) abroad. On top of that, it's not certain that Madrid would want to pay such a hefty fee after bankrolling Ronaldo, Kaka, Benzema et cetera last year, as well as the fact Mourinho says he doesn't want a Rooney type of striker. Again, here we have the same situation as City: a team big on forwards. Higuain, Ronaldo, Kaka, Ozil, De Maria, Benzema, Granero, Canales and Pedro Leon are all competing for four spots. Ronaldo, due to his price tag and status as one of the best footballers on the planet probably has one tied down. Higuain seems to be established as main striker, too. Hence, Rooney would have to battle seven other players for two spots in positions he doesn't favour.
Chelsea: An odd one. Ambramovich has tightened the purse-strings, although could likely be persuaded to shell out big money for Rooney if necessary. A team with Drogba and Rooney in it would be fearsome, no doubt, but would Ancelotti be interested? He's been quiet over it. Chelsea fans have a right to be quietly excited, though.
Barcelona: Unlike their rivals, Barcelona are apparently financially unstable. Whilst their finances have been shrouded in secrecy, there's been rumours that they're, well, broke. On top of that, we have the same kind of problem as Madrid, with a crop of quality strikers. Whilst in this case Pedro, the weakest of the three, would likely drop out of the lineup leaving a juggernaut front three of Villa, Rooney and Messi, Barcelona also have a duty to their fans and philosophy to play Spanish players, and specifically Catalans. Whilst Pedro is from Tenerife, he is also a rising star of the Barcelona team and the world, developing at a scarcely believeable rate. This move is, in my eyes, unlikely.
Below these four in my eyes are the other, smaller teams. Bayern Munich are a big side, but may lack the financial muscle and be restricted by German league rules. Inter, too, may be interested, and city rivals Milan are always contenders for any big signings. Apart from them, Arsenal. Would Wenger be prepared to fork out the cash? He has the money, it is thought, but would be spend it? I honestly don't think he would, but if he did it could be a superb move for Rooney.
tl;dr - I reckon City, if he leaves at all.