The less you put on the 4- position while keeping the 2,5-dimethoxyphenethylamine base, the better psychedelic you get. 2C-D I've never tried, but I've heard nothing but good things about it and my friends have always had great trips at high doses. The halogenated series has the next smallest group on the 4- position, and the lower the atomic mass of that halogen, the better (2C-C is my favorite choice, 2C-B is usually everyone else's, etc.) it is subjectively. When you start putting ethyl groups and thioalkyl groups, you start getting shit like 2C-E and 2C-T-7 which have strong possibilities for dysphoria, vomiting, confusion, hypothermia, etc.
I am not a believer in the 'beginner psychedelic' or 'tofu psychedelic' hypothesis about 2C-C or 2C-D. When you start taking the proper dose (125mg 2C-C, or 50mg insufflated 2C-D, etc.) these are the best and most profound of the 2C-* series, and they come without any of the side effects of the ones with heavier groups on the 4- position.