When I think of Ramen in Vegas, I think of Monta Ramen. It's always packed to the brim with people and there's even more people huddled outside the door waiting to get in. You would think this is the only Ramen game in town.
I'm a fan of the Kuro Ramen which features black garlic oil in the tonkotsu-shoya broth. The restaurant is small and I would not recommend coming with a big group.