In terms of pure enjoyment, my favorite would probably have to be a Denver --> Rocky Mountain N.P. --> Green River, UT --> Reno, NV --> San Francisco --> L.A. via PCH trip I did in the summer of 2014.
But it's hard not to keep coming back to a trip I took in the summer of 2010, when I helped a friend (now my partner) move from Northeastern Ohio to El Paso, Texas. At the time we weren't together in that way, though, and for a number of reasons it was a pretty messy situation that led to a lot of uncomfortable and unpleasant moments on that trip. And yet, it was the first significant road trip I'd undertaken as an adult, and I got to see so much of the country that I hadn't ever seen before, and something about the time of year, the nostalgia-laden infinite promise of summer, made it magical despite the choppy seas. Eventually we got our timing right and we've been together ever since, so now I can look back on that experience and honestly consider it simultaneously both the best and the worst road trip I've ever been on.