After about four years of careful research, I think I'm about ready to suggest some Mexican Restaurants in Japan, let's call this the top 5+! If you have a JR rail pass, you can visit all of these, no problem.
#1: アミーゴ・デ・アミーゴGoogle map
Plaza Del Sol is kind of difficult to find, it's located near Kumamotojo-Shiyakushomae station (near Kumamoto Station), but that won't really help, you really have to print the google map. Food is great, below is the five taco plate, I guess real Mexicans work in the kitchen. Right here: Google map
Casa Del Sol is located just across the street from Sendai Station, about a 5 minute walk from the Shinkansen train. Very good taco's and carne asada, but sometimes they decide to just be closed. So if you were hoping for Mexican food in Sendai but the owner is tired, to bad. Google map.
Taco's on Memorial day!Nice flag.
Casa De Me-gO cefe and bar is about a 10 minute walk from Akita Station in northern Japan. Another place where taco's are really good. Google map. It's called cafe and bar, but it really does not have a bar... But just a after thought of a counter along a wall that is easy to overlook.
Two kind of taco's being served. In the lower right hand corner of the photo was their nacho's, it was kind of messed up. They made nachos the same way a Bistro would make potato gratin.
They even have a harf boiled egg on rice on the menu.
#5: Bar One
This is not far from Utsunomiya station, maybe 20 minute walk, Taco's are okay and the price adds up fast. Google Map.
#6 and #7: I guess this is a rolling-review, I will update as I find new places.
I'm avoiding recommending places in Nagoya and Tokyo, there are already enough reviews available on the internet. Most Mexican restaurants in Japan have the same chronic problem of being overpriced, a typical taco cost over four bucks. The best deal in Japan might be Shooters bar in Nagoya on Tuesday nights, taco's less than two dollars, of course, this is not a Mexican place, so it doesn't count.