Damn right about canned tuna, got to be good stuff, and packed in brine rather rather than oil, the greasy texture and lack of saltiness just isn't right in canned tuna.
Butter, proper, salty, rich butter is the only way to go IMO. I don't buy margarine, won't eat it either, given the choice, disgusting stuff.
Lab supplies. Especially when it comes to glassware and other equipment, I'd sooner spend my money on one set of high quality borosilicate (barring of course, kit used with chemicals that etch or dissolve glass, such as hot, concentrated caustic soda or hydrofluoric acid) glass, and have it last a lifetime, than save a few quid in the short term by buying non-borosilicate glass, or cheap knockoff chinese mass market stuff.
I haven't ever regretted it either, same goes for good quality reagents, where I must buy things from another seller or business, some things of course, are generally unavailable to those outside research and academic settings, although I have some good connections for some hard to source stuff of that sort, some things I still must prepare myself, but nevertheless, quality control is essential, using crappy chemical supplies leads to...well, as they in the computer fields, put garbage in, get garbage out.
Rolling baccy is another one, I only really like amber leaf, after getting used to one brand, others tend to taste like arse.
Cigars..quality only, 'nuff said.