Monta Ramen is the best is Vegas. Hands down. I've searched around to see if any Vegas ramen joints are better and the answer is a resounding no.
The tonkotsu broth simply can't be beat, and the chasu pork atop the ramen is the most tender I've found. I typically add the corn and the hard-boiled egg (nitamago). If you're in the mood for more than just ramen, the kimchi fried rice and the potstickers are both excellent starters.
This place gets crowded. Like, insanely crowded. Line out the door. If you're planning to stop by for lunch, you better be waiting outside before they open at 11:30am. But even if you do have to wait for a table or a counter seat, trust me, it will be worth it.