Wow, wrong time of year to be venturing out to El Centro but if you hafta go, you hafta go...
Top Mexican restaurants would have to include Celia's (near the intersection of Imperial Avenue and West Adams Avenue) which makes all the usual requested favorites (including the "Special Quesadillas" discussed before on these boards) but you might want to order their fish dish. It was grilled just perfectly when I tried it, moist with a great flavor. Also a top spot is Las Salsas on the corner of 4th and Broadway where they make food in small quantities from scratch and have a salsa bar with a variety of salsas. Jalisco's always draws a big crowd but I think the food is mediocre.
A newer, popular restaurant is Junior's that is an old-fashioned diner-style restaurant but it is only open M-F, I think until 3 p.m. It is on Adams Avenue down the street from Celia's. The owner is a professionally trained chef. Mixed reviews on the consistency of the product but he is not back there cooking, just oversees it now (I think!).
The locals love China Palace on Adams Avenue which has a steak house on the other half of the building. Steaks are nicely prepared but the prices are high for what you get.
The new mall has a lot of the national chain restaurants like Dave's BBQ, Johnny Carino's, Chili's, etc. Nothing spectacular there... There is a longtime Italian restaurant, Grasso's, on West Main Street. Not many of those in town, so if you are stuck there for awhile on business...you're gonna have to go there for Italian (or Carino's).
But, the Number 1 restaurant in El Centro in popularity is undoubtedly Burgers and Beer. It is the social center of town, it seems. Good burgers and a variety of Mexican favorites on the menu. When anything is going on in the Valley (Dove Hunting, Rodeo, etc.), this is where they all seem to end up.
Anyone who has come into town and tried to bring an upscale restaurant to town has failed. The locals just won't support it. That's not to say there isn't good food there, but it would be hard for me to tell you what to order on what restaurant's menu because things "overall" aren't usually consistent.
I don't know how many El Centrans are CH so I hope you get some good rec's and please report back. It's been a long time since I've lived there and these suggestions are from some sporadic visits there recently...
I don't know restaurants in El Centro, but I would recommend that you bring a cooler (presuming you're roadtripping from SD, which may be wrong) and grab some pre-marinated but uncooked carne asada and/or pollo asado from Kennedy's Karne. Get good tortillas there while you're at it.
However, if you live in/near Escondido, never mind the cooler, because Kennedy's Karne has a place there now too. And that one serves lunch, but I don't think the one in El Centro does.
Sorry I couldn't be of much help on the original restaurant question, but did want to share this tidbit in case it helps...