I have always considered that you pay for texture and not flavour. A properly matured rump steak cooked simply with a 4 pepper grind is just incredible flavour. I slow cook a lot because we are either both out at work, or she is, and i'm playing AH

I use skirt and chuck or the very cheapest cuts for curries or bourgignons. Seal them at atomic temperature and then cook them for at least 8 hours.
Not long ago I did a shoulder of Venison. Marinaded in port and red wine for 48 hours, sealed, then into the big slow cooker for 12 with beef stock, the marinade, more port, mushrooms redcurrants and shallots. That was goooood