OP, one thing you need to consider is that depending on your starting resources, each person's best resource set may be different. The 3 BR set is the best one, and unfortunately, the golden rule of CN trading is as follows:
Your starting resources, Oil / Gold, automatically put you into the 5 BR category for best setup. It's not quite as good as 3 BR, but close enough to work well. Keep in mind it only makes you money if you buy lots of infra, however, because it's infra price and upkeep reduction heavy. Your optimal set includes:
Aluminum | Cattle | Coal | Fish | Gold | Iron | Lumber | Marble | Oil | Rubber | Water | Wheat
This set will give you Steel, Automobiles, Beer, Construction, and Asphalt. I've been doing trades for quite a while (which earned me the nickname of "the Trade Guildmaster" during my time in LoSS) so you can trust me with this assessment, however, if you'd rather do the math, feel free, but you should come up with this same set eventually.
Never underestimate the power of population bonuses. Although having tons of bonus resources looks nice, realize that the resources required to get them are far inferior to the ones required for other sets. 3BR > 5BR > 8BR