Realistically, what is the biggest franchise that Microsoft can buy out that would give them first place in console sales?

[Burek]

There is no realistic option right now.
I think MS would be better off developing a killer franchise by themselves, and who knows, maybe secretly they have been doing that and will blow everyone away at E3 2015....but I doubt it.

An existing franchise that could turn it around and propel them to #1 would be way too expensive, guatanteeing unprofitability for XBox forever.