Of those listed I have tried all except 2C-N, 2C-T-4 and 2C-T-21. And 2C-T-7 I have never taken a full dose of so I can't really say much about it.
I have chosen 2C-D as my favourite because it is the one I find the most entertaining and euphoric. It is insightful and nootropic as well as a little speedy, a bit toy-like if you will, at lower doses and a benign yet emotionally very deep psychedelic at higher doses.
For different reasons I also very much like 2C-B and 2C-I, 2C-B being a very sensual experience and 2C-I being a very humorous one... both of these I consider to be potentially very social and I'd rather take them in company than alone - I feel that when I am alone they can be more confusing than most other psychedelics and can somehow turn on me and give me a hard time.
2C-E I feel has much potential, yet it is a bit long-lasting and 'dyed-in-the-wool' psychedelic for me to do it often, though I definitely wouldn' t want to not have it at my disposal because of thereasons Xorkoth has already mentioned ... but at this point I am more inclined to use LSD for that purpose than 2C-E
2C-C I have found less satisfactory than any of the other 2C's I have just mentioned although by all means I still consider it a worthwhile and not too difficult experience.
2C-P and 2C-T-2 I have both found a bit too hard on the body to try them a second time, although both have some potential and I would consider ingesting again if set and setting deem it favourable. 2C-P is valuable for insight into actual thought processes, whereas 2C-T-2 gave me an unusual euphoria that felt like speedy MDA or something.
2C-TFM I do not know for sure if I had any that was pure 2C-TFM, though I think I may have. I found it toy-like and the cognitive enhancement very enjoyable and would almost rank it along with my favourite, 2C-D, on the top my list if a later trial with the contaminated batch hadn't given me some body issues. 2C-TFM, like 2C-D, I find can be taken when focus on one task is wanted as well as for tripping in company.
As such I too believe the poll to be flawed for the reasons mentioned already.
Also, my preferences expressed here are likely just a slice in time and I may change my opinion on any of these substances over time due to my personal evolution as well as due to further experiments with said substances.
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thinking about what I just wrote, I'd like to add that actually, the question I have answered isn't "What's your favourite 2C?" but rather "Which 2C are you most likely to take in the near future?" . . . that kinda sums it up and that kinda says which is my favourite but only RIGHT NOW, really. When I am more inclined to do self-exploration again rather than taking them recreationally as I tend to do at the moment, it may be one like 2C-P or 2C-E. But I don't see as much recreation in them (in terms of recovery/integration time on a number of levels) and so I have to wait for the right time in my life to take them as they really have few if any of the toy-like mindcandy aspects of those I am most likely to take in the near future.