I honestly change my approach daily after reviewing the turn ins available after reset. Whichever requires 3 or less materials and gives between 50-80 red scrips usually are the ones I go for. However, I find a lot of success with Noble Gold/Codex, Buckwheat Tea, Koppranickel Ornamental Hammer and Beech Composite Bow. The name of the last one may be slightly off but it should give you an idea.

My retainers go into overtime when I farm red scrips, and you are making a very smart move. I haven't bought a single materia off the MB (the costs are ridiculous, like even 9K for a CP+1!) and I try to limit my spending for mats off the MB. remember too that it can reimburse you sort of, if you get full collectability, all the ones I mentioned above give about 1,900-2,800 Gil per turn in.