Here's my personal list. Some of the burrito places can vary by location, and the Chilango locations in the City can really slip during the lunch rush.
Sit-Down Mexican
1. Meztizo (Hampstead Road)
2. Wahacca (Multiple Locations)
3. Cantina Laredo (Covent Garden, a US import)
4. Taqueria (Westbourne Grove)
5. El Camion - Used to be El Camino (Soho & Portobello Road)
Burritos
1. Daddy Donkey (Leather Lane Market)
2. Chilango (Multiple Locations)
3. Tortilla (Multiple Locations)
4. Benito's Hat (Multiple Locations, including KX Station)
5. Chipotle (Multiple Locations)
6. Adobo (High Holborn)
7. Luardo's (Whitecross Street Market & eat.st)
Do Not Eat List
-Chiquito's
-Texas Embassy
-Las Iquanas (though its more Latin American)