IMHO, the best nachos on WDW property can be found at the Pepper Market at Coronado Springs Report. I've been craving them for weeks, and I can't wait the 89 days until my trip to go down and have them.
I believe they also serve taco salad, burritos, etc. in addition to the nachos.
The Mexico pavilion also serves them. For prices, check www.allearsnet.com for menus and costs.
I was disappointed in the nachos at the Cantina In Mexico. They werent "bad", but they only had that cheese sauce on them(like you get at sports stadiums),not real melted cheese,and for me that makes or breaks nachos.
Ive found the ones at ESPN to be very good! I get them with just the cheese and beef on them, and the sour cream and pico de gallo on the side. Yes, Im difficult, but they are worth it.
If WC is crowded, or you don't feel like the crazy atmosphere that can be at WC. You can also get the same nachos at the Lounge next to Artist Point. This is a nice lounge. The nachos at Roaring Forks will just be the cheese sauce like at movies