I have flown Jet Airways a few times between BOM and LHR and found it to be the best for comfort in Economy. The seat pitch is generous, and the seat design lets the bottom slide forward when you recline the back, giving you a slightly better position.
For seat selection, I have gone for the very last seats in Economy - 61A or K as these are the only double seats and you get a bit extra room between the seat and the aircraft side, plus the seat next to you is not officially an "aisle" seat so you have a good chance of it not being occupied. Drawbacks of these seats are proximity to the rear washrooms on the 61K side - the light when the washroom door is opened tends to shine directly on you; and if the flight is not completely full, you lose the chance to get a full row of seats to lie down on. Row 61 also may not have a window on some aircraft.
Another option is right at the front of economy in row 30 at the emergency exits. You get plenty of legroom although drawbacks here are that the window seats are a bit cramped due to the escape chute housing encroaching on your leg space, and people tend to hang around that area waiting for the washroom.
Usually, I just take pot luck and have had no problems, especially if I check in early (in BOM) or online (in UK).