Pitch Black 2 was the absolute best. But that was only around one Halloween season, back in 2005 I believe. The original Pitch Black is great too, but that sour version of Pitch Black 2 could never be beat. I was so happy when Pitch Black became a permanent flavor, and then really bummed to see it end up being removed from the permanent roster like 2 years later.
Other favorites are also no longer around: Typhoon was amazing, and it narrowly lost out to White Out to be the next permanent flavor at the time, like as I recall it was incredibly close. White Out is fine, like a Mountain Dew caffeinated version of Fresca, but Typhoon was a brilliant tropical punch, that ranks among the best flavors. I believe I read it will be coming back for a limited time at some point, so I'm excited if that's the case. And then there is Sangria Blast, Taco Bell briefly had a second flavor to compliment Baja Blast a few years back, and it was even put in bottles for a summer, but then poof, it's completely disappeared, which is a shame, cuz it was a good tart, pomegranate flavor; which I wouldn't think would be something I'd be clamoring for, but damn it worked, and I loved it.
Oh and a new yearly favorite has become Liberty Brew, that better be back again this summer, that flavor is addictive as shit lol.
In terms of what is around today permanently that is my favorite, then it's Livewire. By far the best orange soda there has ever been, do not try to convince me otherwise. I also really like Voltage, but that's the one Mountain Dew flavor that I can only have in small doses, for some reason that flavor in particular has a knack for causing me gut-rot, and I'm just never in the mood for dealing with that.