#1 Andrade "Cien" Almas

During his call-up to the main roster, WWE advertised him as the top star, the one everybody needs to watch out for, this is actually true. He has literally won dozens of championships with many top promotions prior to joining WWE.
After coming to the main roster in May, he had feuds with Sin Cara and Rusev, which were excellent in terms of in-ring action.
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Right now he and Vega are having a feud with Daniel Bryan and Brie Bella which will probably not go a long distance as Daniel is involved in a feud with Miz, and is scheduled to face him in Hell in a Cell, in a mixed tag team match. So basically, Almas will be without any opponent for Hell in a Cell.
Nakamura will be the perfect match for him, as both are great wrestlers, and fans will be genuinely invested in their feud. Imagine the show they would put up. It will be a dream match for me, add the US Title to the picture - it would be icing on the cake.