The Leela Delhi is brand new, so I have yet to see it, but the group is very well known and respected in India and service is generally extremely good.

Personally, I would avoid The Imperial as although it looks good, for me it does not work on a service/food level. A very attractive, historic property, though.

Taj Mahal Hotel has excellent service & food but rooms are a little dated now.

The Oberoi Delhi is not as good as The Oberoi Mumbai and can be a little bland in places - too much of a business hotel. Service, in my experience, fluctuates from great to OK.

Aman Delhi, although not perfect (nothing is in Delhi) is a great experience as it is a unique hotel for India - very Indian yet very contemporary. The entry-level rooms (all with private pools on the balcony) are by far the nicest in this city and they have by far the best spa. Great GMs, too. This is where I currently stay, but I would not be unhappy at Taj or Oberoi.