#1 George St. Pierre

Conor McGregor vs George St. Pierre is probably the most marquee fight UFC has available to them.
A first-time fight between the legendary pair would undoubtedly be a huge pay per view draw and could even set an organisation record for pay per view orders.
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Pierre, like McGregor is a former two-weight UFC Champion, having won gold at Welterweight (twice) and Middleweight. McGregor has previously reigned as Featherweight and Lightweight Champion.
Pierre would likely need to drop weight to make the fight happen but the Canadian has shown willingness to do just that and has openly spoken of his desire to fight "Notorious."
St. Pierre has trimmed down from his one and only fight at Middleweight in November of last year and could likely drop down to the 155 Ibs required to fight McGregor.
This fight has to happen and there is a good chance it will in 2019.