Some recent recommendations are here:

http://www.fodors.com/community/asia/india-travel-agent-recommendations.cfm

i have had very good luck twice with castle and king in delhi..

google them

In three weeks you are planning to visit both north and south India? Let me suggest you choose one or the other for your first three-week visit.

Faith at Indian Panorama [faith@indianpanorama.in]gave us wonderful help in planning a 4 week trip to both Southern and Northern India. She is easy to work with, quick to reply. The drivers we had from her company were outstanding and hotel selections great.

agree 1000% with above. I've booked two great trips (and part of a third) with Indianpanorama. Do be sure Faith herself checks the final itinerary.

We ended up using INB and were very satisfied with the work of Mr. Thakur and his staff. All arrangements were as promised and provided in a timely manner. We requested three star hotels and were generally pleased with the quality of our accommodations. We would recommend INB if you are looking for reasonably priced and efficient service. http://www.inbvacations.com/ Our trip included Delhi, Varanasi, Khjuraho, Orchha, Gwalior, Agra, Fatehpur Sikri, Jaipur and Udaipur over 17 days.

We used VP Singh, LegendsandPalaces.com.
He didn't just show and describe each place visited; he helped make everything come alive.
He is an interesting, intelligent and fascinating person.
My wife and highly recommend him.
He will work with you on an itinerary that will introduce you to India and immerse you in their culture.
Have a great trip.