To me, the ultimate taco is the Funkmeister at Moe's Southwestern Grill: pinto beans, meat*, cheese, and lettuce on a soft-shell taco...perfection.
Johnathan
*They have steak, chicken, ground beef, and catfish...I've only ever had the first three, and while the ground beef was pretty standard (nothing special), the chicken taco was the best chicken taco I've ever had in my life, and the steak taco was the best steak taco I've ever had in my life.