As far as 'worst amps' go it's obviously the Carlsboro thing was was at a mate's house back in the '90s, but in a very close second place would be the Ashdown ABM300 that I bought (when the transformer on my Ampeg SVP blew) brand new and the buttons kept falling apart (literally split when I pressed it) so I returned it.
In its defence it sounded GREAT -really clean, punchy DI sound with tons of 'heft', but I didn't have any faith in the build quality.
I also had a Trace 150W GP7 head that sounded great but couldn't keep up with my drummer at the time, but maybe I was expecting too much from it.