Sleeman's not bad. It's certainly better than generic Molson or Labatt crap and doesn't deserve to be in the infamous company of Busch, Pabst and Schlitz. Most Sleeman varieties are at least drinkable, although their new "low carb" is supposed to be nasty.
I'm going to limit my choices for "worst beer" to beers which I have actually dared to drink. I'm a fussy beer drinker and I've avoided plenty of low-brow stuff (like "malt liquor", the "dry" craze, the cheap "strong ale" craze and now the "low carb beer" craze).
Just to let you know my prejudices - I'm a Canadian who's partial to British beers, Belgian beers, Mexican cervesas (sp?) and Canadian microbrews. I'm underwhelmed by Canada's big name beers and generally repulsed by American big name beers (although some US microbrews are quite nice).
Worst Canadian Beer (that I've actually tasted): Black Label (horrid aftertaste)
Worst American Beer (that I've actually tasted): Budweiser (godawful stuff - oddly, the version of the stuff brewed under licence for the Canadian market is only nasty tasting rather than outright vile)
Worst European Beer (that I've actually tasted): toss up between Grolsch and Kronenbourg... both are just vile tasting...
After a bit of thought, I'd have to say Kronenbourg was the most offensive beer to ever hit my palate. I'm sure somebody out there must appreciate it, but I can still conjure up its strong nasty taste and lingering aftertaste 15 years after the first and last time I drank it... ugh...