Okay, well redtailes are BEAUTIFUL snakes but i havent heard of them being really nippy at all.. apparently boas can be very soft haha. But yes, at 12ft max they are stunning snakes so long are you willing to take it on, which im so pleased to see that you are.
Ball pythons wont actually grow that big and even though they come in loads of different morphs they are also very soppy snakes.
I don't know anything about bull snakes sorry ):
Emerald tree boas are very pretty too, but one thing i get told very often is that these snakes are more for display than handling. They WILL bite and could keep the attitude for a very long time. The don't move a lot and aren't really great pets to hold.
Also i don't know a lot about yellow anacondas but they are very pretty too.
Something you didn't mention is a carpet python. I have a jungle carpet python and trust me, shes nippy all right!!! they grow up to be striking yellow with bold black stripes and although they calm down after a year, you still have to put the effort in while they nip your fingers off haha.
Its kinda cute though. Jungles are notorious for being nippy, read any care sheet.
the females can grow to be 8ft max so a lovely size and not too thick as they are slender bodied.
http://wyckedwytchinme.deviantart.com/art/Me-And-Taboo-93627517 Heres a great size example.
But sorry for going off topic a bit. If it were me id go for the redtail boa... and i couldn't chose between the rest. Im a fan of the carpet pythons really haha.
p.s - I know someone selling jungles still (sorry UK only), same breeder i got mine off. But again, you might not be interested in these snakes.
Hope i helped! good luck with these snakes <3