Seems like .Co is a good balance between all of them if you can't have the .com
I agree with everything you said expect the part about .co. Stay away from it. Looks like a .com typo and will be forgotten eventually IMHO
That's the only thing I don't like about .co. Every time I see a .co name on a van I always wonder if the M fell off.
lol .. don't dismiss .co too quickly .. the reason I buy some is because I've specifically seen them out in the world .. on TV commercials, billboards and other forms of advertising. Like every other TLD, it most definitely is a small fraction of .com .. but it is stronger than most gnTLD's.
Here's how I answer your generic question (not bitcoin specific):
.io = Tech (Industry facing B2B)
.net = Tech (Consumer facing)
.co = Good generic
.biz = Generic that isn't as bad a choice as people think
.org = Authority website (if you want too look like an official entity)
.info = Information/wiki website (if actual info is more important than brand).
ccTLD = Obviously if it's not international then the country code is a good option.
Obviously one important factor to consider as well is if there is an industry standard as mentioned above.
(@Nikul Sanghvi, @atinc, @Duncan Burgess)
As for your specific BTCBroker domain .. the best fit is .net .. although if the industry trend is .org or .io then go for that. That being said, .net is weak at the moment .. if your goal is to resell at retail then you could be sitting on it for a while. You might be able to quick flip at 2x-4x regfee to another domainer (BIG maybe). I'd personally only grab it if you were actually planning to develop out the site yourself.
Keep in mind I'm not into bitcoin and only have a couple crypto domains .. my post is based on general observations!