Maverick
Having read a few (Chickenhawk, Low Level Hell, Snake Pilot etc) I'd recommend 'Maverick' by Dennis Marvicsin and Jerold Greenfield as the best. Absolutely brilliant if you really want to get inside the head of someone going through Vietnam as a helo pilot. Marvicsin did 2 tours (Huey's & Cobras) and ended up as a POW, quite a story and very engagingly written.
Other than that, Low Level Hell is well up there.