I am not a proponent of SLI, so picking the single-fastest GPU possible is ideal. No card comes close to the 1080TI besides the TITAN X. For the sake of better NVEC and other features, the 1080 TI wins over by a longshot.
Good luck finding a 1080 TI right now however.
We have a listing of all of the 1080 models available here: http://legacygg.com/links/category/12-top-of-the-line-cards/