Bill the Butcher and the Dude are the two figures I think will hold their value best from Rainman due to the sheer quality of the figures , the fan base of the characters and the small chances of competition in 1/6 with them ( especially of this quality ). I'd say the same for the fight club set but with Iminime already planning a Tyler and possibly a Norton and so many custom figures and sculpts being around it opens up more options for interested buyers. Not to say they will beat the quality of Rainman's figures or course and definitely not to say they will drop below retail for a long time , that goes for the rest of his figures also.
I was never interested in a Bill figure since I'm not a big fan of Gangs of New York as a movie , but with how amazing the figure ended up , how rare it is now and the fact I wasn't happy with the clothing set of the Daniel Plainview I was waiting for ( couldn't afford the full set at the time ) I do regret not picking one up.
Like many of you here I've been in this hobby since collecting Mcfarlane stuff and it's come a long way.