I became full time vegetarian in 2016. Yes, I was half time vegetarian earlier :)
Most of the non-veg restaurants also serve veg dishes.
But if one is specific about eating only in vegetarian restaurant, then there are some prominent names.
First is Sagar Ratna. Excellent food, good ambiance, courteous staff, clean environment. I have been to Sagar Ratna at more than 5 places and found uniform discipline in all their restaurants. They never compromise on their quality. They have sitting arrangements.
Second name is Sarvana. They have different culture than Sagar Ratna. Food is good but sometimes their staff seem to be in hurry to bring in next customer on your table. But this was my personal experience. Experience may vary person to person. They have sitting arrangements.
Third name is Haldiram's. Good ambiance, good food, wide variety. Great combo of sweets and food. They have sitting arrangements.
Fourth name is Kaleva, at Gole Market. Not a very big restaurant, but neat and clean, good food, wide variety of sweets and food. They don't have sitting arrangement, but they have placed tables. You can stand and have your meal comfortably. I visit Kaleva once a month after visiting Kali Mandir which is situated nearby.
Fifth name is Kadimi, Janakpuri. Again not a very big restaurant, but they have nice variety of sweets and food. They have sitting arrangements. Neat and clean restaurant.
Bengal Sweet home at South Extension, Part 1. Big restaurant with nice seating arrangement. Nice food and neat and clean place.
Dilli Haat, near INA. You will find great food from different states because this is what Dilli Haat is all about. Nice food and handicrafts.