That's a highly subjective question. Personally, I look for weed that has a distinctive, strong taste (my personal favorite for taste/smell is Sour Diesel). I also look for something that will actually overcome my tolerance level- The NorCal purple stuff is nice- Grand Daddy Purple will rock your world.
I wanna try some Bubba Kush, that's supposed to really be something else as a medicinal smoke.
Hey, in the end, 75% of the fun is in the actual hunt for the best, right?