I've tried a lot of Mexican places, and IMHO, the absolute two best in London are Green & Red in Shoreditch, and
Mestizo by Euston. I also like Wahaca, but it's kinda more like a Mexican Wagamamas - nice, fresh food, but in a "rush in, rush out, we need your table NOW" atmosphere rather than a proper restaurant.
El Pacifico and La Perla in Covent Garden are both okay, though more leaning onto the Text-Mex side of things, and I
hate the Texas Embassy - there is precisely one thing on the menu NOT fried and smothered in cheese (the chicken breast swimming in bottled BBQ sauce). It's Texan fried cheese heartburn alley, not Mexican food. I feel roughly the same way about Chiquitos.
Also, I wish to god I knew the name of the Mexican place we ate at in Edinburgh a couple years ago, but I was on a hen weekend and someone else did the booking. Every single one of the English girls ordered the fajitas (I think they were scared of the unfamilar menu) but I always order the mole if it's on the menu and it was SUPERB there. The waiter also took a shine to me because my pathetic high school Spanish actually came back to me and I could pronounce things properly.
(In a slight thread diversion, anyone know of anywhere brick & mortar in London I can buy Masa Harina? We ran out and with the postal strikes, I don't want to order online)