I love the Brooks Brothers Extra Slim Fit, and they can be had on clearance for $39 a shirt. But with an 18 inch neck that might not be the ticket. As far as most queries go, for slim fitting OTR shirts, I think they are the best buy.
I've got BB Extra slim, CT tailored fit and some pre-SF slim Banana Republic shirts that I like. I also have a couple MTM shirt from modern tailor. I based the measruements off the three I just mentioned and basically got a nearly perfect shirt first time around. You could try getting the measurements from a good fitting shirt you have. Have it tailored if it doesnt fill well already.
