There are multiple trains which run only between Delhi and Dehradun (link: IRCTC). Nanda Devi express can be given the highest preference. It runs on time mostly.
The best buses from Delhi to Dehradun are of Uttarakhand Government (link: UTC online). They are comfortable and on time. Timings for Volvo AC buses are:
Note that there are other categories of buses too. Volvo AC buses are the most comfortable ones.
Direct flights (0 stop) to Dehradun are from Delhi, Mumbai, Lucknow, and Bengaluru. Flights from other cities are via Delhi or Lucknow mostly. A flight from Delhi to Dehradun is of about 1 hour’s duration. The first flight leaves at about 6:10 am.
The airport is 31 km from Dehradun railway station and about 17 km from Rishikesh. Rishikesh and Dehradun are in opposite directions from the airport. Prepaid taxis operate from the airport and can be booked in their office outside the airport building.