Honestly I can't think of a single piece of gear I regret buying.
I would nominate M-Tron software as a dud for the simple fact that the samples are hissy beyond use-ability.
Oh, and I guess I should mention Groove Tube "Mullard" re-issues. Those were shit straight out of the box, and Aspen Pittman is a con artist of the highest order IMHO for unleashing that shit on the buying public.
But other than those small exceptions, I'm pretty damned satisfied with the quality and durability of the gear I have gone with over the years. Moog, Rivera, Victoria, Fender, Gibson, Martin, Lynx, Event, Carver, Rode, BLUE, Naiant, Sennheiser, boutique stompboxes--- none of them have ever let me down.
If you stick with the major players and use due diligence before laying out the cash --and don't have unrealistic expectations-- you can't go very far wrong in most cases.