MS is an obvious choice, but Epic have also got themselves contractually supplying their engine to a whole host of developers, many of them producing multi-platform games, which I suspect would horribly complicate such a purchase with regards to on-going support for those clients.
Also, Activision or EA would surely be interested, and overall represent even more obvious purchasers with the ability to more easily accommodate Epic's middleware business as well as leverage the fact that Unreal is a multiplatform engine.
This does illustrate the continuing trend for consolidation and the absorption of smaller companies into large global businesses.
Try to be reasonable... its easier than you think...