I would choose Air India as I am always flying whit national airlines as my first choice and I like their service, I have never been flying whit the other options.
I have to fly this route a few times a year for work, I've tried all the airlines that fly the route except Spicejet so cant comment on them, its a short flight and you don't get much really on any of the carriers.
The last 2 times I went with Nepal Airlines and found them to be quite good, I also haven't had any issues flying Jet Airways in India, I find them quite reliable and less chaotic than Air India... I tend to avoid Air India, I've had nothing but trouble with them! Indigo Airlines also fly DEL-KTM but are a low cost carrier, you have to pay for seat assignment and meal, however I have flown with them domestically within India and have to say I was quite impressed with them, planes were clean, cabin crew were young and friendly... unlike Air India!....maybe an option if your on a budget or the others are not available.
I guess overall I would pick either Jet Airways first as they have 3 flights a day then Nepal Airlines,