Koukouvagia -- It appears you don't know exactly what you want -- which is fine.
There's a style of Nachos -- perhaps the original -- which is simply cheese melted on chips. Obviously, the cheese will re-solidify very quickly if that's all you do. I don't think that's what you want though, or you'd already be doing it.
You can build a platform for experimenting with different cheeses and flavor profiles by starting with either a fondue recipe or beginning as you would for making "creamy" style macaroni. Rarebit recipes might be another productive path -- I certainly like the idea of beer in the Nachos.
The first uses a flour or starch based slurry and continuing heat to hold its texture; the second, a roux based bechamel (dairy) or veloute (stock).
Using any of the three techniques you're looking for cheeses with good melting properties.
Because you haven't spoken about your desired flavored profile(s), I can't make suggestions about cheeses other than to say (1) I don't think you'll get any benefit from Mexican cheeses (such as "Chihuahua" or "Oaxaca") which you can't get from more commonly available and less expensive cheeses like "Monterey jack" for instance; (2) the typical "Nacho" cheese flavor is very close to cheddar/jack.
FWIW, that's not a recommendation, only an FYI.
Why cream cheese?
Just as a throwaway towards a very basic and typical "Nacho" with some good technique twists: Start by browning some onions lightly along with some fire roasted, peeled seeded and coarsely chopped jalapenos; add flour and cook the raw off it. Add a chopped canned chipotle (for smokiness, sweetness and heat), along with some of the adobo (for complexity). Add chicken stock with a healthy splash of beer, pinch of dry or dijon mustard (cheese loves mustard), raise the heat to max and cook until the "veloute" completely thickens (to about nappe stiffness) -- usually about three minutes on the boil, stirring (or whisking) as you go. Reduce to a bare simmer, add grated pepper jack, and a decent sharp cheddar (50/50), and stir (or whisk) as it melts to keep everything smooth. When the cheese melted, I'd season with a few splashes of hot sauce (to taste) and stir in a bit of chopped cilantro for color and freshness.
If you're sharing with someone who doesn't like chili heat, limit or omit the jalapeno and/or substitute fire-roasted and peeled poblano for it.
Hope this helps,
BDL
Edited by boar_d_laze - 6/15/12 at 8:14am