I've never found the perfect combo. I know a common Tex-mex cheese is Land-o-lakes Extra Melt. It is usually a restaurant only product unless you're willing to buy a 30 pound case for nearly $300. It is also the standard for most quesos.
Often recommended to target the LOL stuff is Kraft Deli Deluxe. If you do that only it has what I consider to be too strong of an American cheese flavor - that odd overly processed bite you also get with velveeta.
I tend to agree with a mix of cheddar and something else. My personal preference is stronger than mild cheddar, but that could be because I've yet to nail the tortilla prep process where they don't end up mushy. Once mushy the cheese and tortilla taste like a single substance. I've been experimenting cheddar/deli deluxe mixtures.
For reference I like the cheese used in Mia's, Mi Cocina (probably identical to Mia's), and blue goose. Those to me all taste closer to cheddar than generic processed cheese product.
Staff - take out the trash.