Hi!
This year I've been a second time in Rajasthan because I missed Udaipur last time.
To make it short:
Don't stay overnight in Agra, a day trip from Delhi is absolutely enough! I met nobody yet who liked that place (besides the taj of course).
I have been in Varanasi and enjoyed my stay but I heard from fellow travellers that you can have a hard time if you're not in walking disance from the Ganga because of the rickshaw drivers who have a real mafia there. We went to the tourist buro at the railway station to contact our guest house (yogi's) to send somebody to pick us up. The tourist buro also organizes city tours.
I stayed a certain time in Jaipur (It's ok to miss it but it's definetely better than Delhi!) and did a weekend trip to pushkar and Ajmer. We did not even stay a whole day there! It is very similar to Udaipur but in the last there's less hasseling for donations. Anyway if you go there don't miss the old mosque 'three days hut' near the main Durgh in Ajmer (300m left when you come out). Really peaceful contemplative place.
My favourites are Jaisalmer (I stayed there two month because the people were so nice! Check out Shahi palace hotel www.shahipalacehotel.com) and Udaipur (the lake is simply great!)
I wish you a great time!