I have been to a lot of restaurants in and around Delhi but I am yet to find the one that I can say is the one I would like to go to again and again. I will have to admit that I haven't been to any high end restaurants or the ones that you could say are posh in any sense so I am now willing to explore a bit pricey options in search of a good restaurant that I can recommend to my friends who visit my in India or take them out when we go out. The ones I have been going to are all run of the mill restaurants that do not belong to a chain and to make things worse they have all been vegetarian restaurants so far. I have returned back to my carnivorous ways again and I am craving for some good meat which is hard to come by where I live. I am open to suggestions all across Delhi and even NCR as long as it does not cost me an arm and a leg.
I replied to another similar thread the other day which will answer all your queries about best restaurants in Delhi. I hope this helps but if you have more questions then please feel free to ask.
Haldiram's is a good food chain, I wouldn't say they serve the best sandwiches or pasta's, but their Indian food is very good and very hygienic, which is why the cost is slightly higher than other places, but defiantly worth a try. For South Indian food I would recommend Sagar Ratna, you serve authentic south Indian food and have a lot of variety in south and north Indian cuisines.
According to me there are a select few places which you can calls Delhi's best. The first is Haldirams's, Karims, Moti Mahal, Park Balluchi and I would not call it a restaurant but a food outlet, it would be kulfi falooda and there is another dish from Chandni Chowk which is called dault ram ki chaat and that is heavenly (unfortunately I don't know which food outlet it was from).
Mainland China is the best place to have authentic Chinese food, the cost is slightly on the higher side but worth it if you go once in a while. Other restaurants in Delhi include Kareems which is in Chandni Chowk.