Bairaktaris is one of the most known "souvlaki eateries" in Athens. He's been so successful that over the last 20-30 years or so, he slowly purchased most, if not all such places in the wider Monastiraki area. Food-wise I would certainly recommend it for that line of food, but the rest, especially the non-grilled stuff, is not made in the store but in some centralized kitchen. Still, people and especially tourists still go there.