Hi! Could anyone advise a place to have breakfast in Mexico City?

Where in Mexico City?

Right down the block from the Gran Hotel Ciudad de México there's a great hole-in-the-wall on Avenue 16 Septiembre. Across the street there's a VIP (chain).

Mexico City is big place. How about a clue as to where you want to have breakfast?

Assuming you're in centro, until you indicate otherwise, I can recommend 2 places. Sanborn's in the Casa De Los Azulejos, is absolutely amazing. The ambiance more so than for the food. Though the food is still pretty good for a chain.
https://en.wikipedia.org/wiki/Casa_de_los_Azulejos
The other is Cafe Tacuba, the restaurant, not the rock band. A CDMX classic.
Restaurante Café de Tacuba | Desde 1912
We normally don't eat breakfast out, preferring to opt for just coffee and a bakery good or 2 in our room before heading out. But we have been to those places. But there are other classic cafes in centro.

Also a fan of Sanborn's here, especially that one.

That Sanborn's, in the "House of Tiles" is worth visiting. Similarly, in the historic center I've had a good breakfast at Bisquets Obregon, located at #85, Calle de Tacuba.

On my high school Spanish class trip, decades ago -- we won't say how many decades it was -- we had breakfast at that Sanborn's every morning in CDMX. It was pleasant, reassuring surroundings for a group of wide-eyed high school kids.

We ate there for breakfast when we were in Mexico City several years back. It was very cool.

I don't like sit down breakfast most days and also do the pastry and coffee thing. There are some excellent panaderias. From one of the street vendors around my hotel, I also sampled some champurrado or atole. If you've never had it, give it a try.

It would help if we knew where OP was staying but they haven't come back to say.

Hey, many thanks for the answers. For the best breakfast in the city I'd go anywhere