That's kind of a tie between a Kirby G3 I got at Goodwill for $10 and a Filter Queen Majestic I got at a different Goodwill for $5. I really regret not walking away from both of them.
The Kirby's bag was stained and discolored, and there was a huge amount of black mold inside the motor housing. When I found that, I immediately took it outside, put on an N95 mask, and blew the hell out of it with my air compressor, then doused it with Lysol (probably should have lit a match at this point) and let it sit open for the rest of the day. The brush roll's bearings were also completely frozen, and the tech drive would only go in reverse. I had to go caveman on it with a hammer to knock the sliding mechanism loose where it would work properly.
The Filter Queen had enough dog fur in it to knit another pooch, and it reeked of that and dog piss so badly that I never could completely get the stench out, despite giving it the deepest deep cleaning I've ever given a vacuum. I even filled the barrel part with a solution of water and Pine-Sol and scrubbed it with a toilet brush. By the time I got it all back together, after spending $50 or $60 on parts, its performance was mediocre at best, so I gave up and relegated it to the barn for about five years before donating it to the church's rummage sale.