i voted 6-8, but i also switch it up, as should everyone!
when buliing, i use 6-10 because that works best for me in terms of mass.
however, when training for strength, i prefer the range of 3-4 for larger muscle groups(chest, back and legs) and 5-7 for smaller(arms, delts and calves)
I'm sure i have already done a poll incredibly like this one.....hmmmm
Right now I'm doing H.I.T., one set to failure on everything. 2 Exercises per bodypart. Very strict form-using a 4/4 count. Reps are at least 12 - but I go to complete failure!!!