The Avengers have one of the most intense rogues galleries to exist in comic books; after all, as the legend goes the team were brought together to defeat the foe no single superhero can withstand.
When the team first got together, that was the Hulk, brainwashed by Loki. Since then, the threats have gotten bigger and badder - even Loki's powers and personality grew to make him more formidable - and there are times at which not even the combined might of the Avengers is enough to win the day.
Of course, this being comics, they have to, so no matter how powerful these villains might be, they'll always be trumped in the end. That doesn't mean that our favourite heroes don't have a tremendously difficult time fighting them - often dying in the process.
Honourable Mention: Baron Zemo & The Masters Of Evil
These guys don't quite make the list because after all, there are several of them. That's cheating, guys.
Zemo, a fairly prolific Captain America villain, is pretty much just a really intelligent bad guy with a sword. That said, his charisma as a leader drew a number of like-minded self-proclaimed evil types to his side. Originally the line-up consisted of the not-inconsiderable foes (new name for a supervillain team - The Not-Inconsiderable Foes) Radioactive Man, the Melter and the Black Knight.
Quite the prickly thorn in the side of the Avengers, Zemo eventually returned with Asgardians Enchantress and the Executioner at his side, but his own crazed desire to kill Captain America resulted in some poor strategic choices - landing him buried under a rockslide. Ray guns don't melt Vibranium shields.
His son would later return with an army big enough to invade Liechtenstein, including such notable members as Absorbing Man, Moonstone, Mister Hyde and the Wrecking Crew. They even managed to invade Avengers Mansion, taking it over and severely endangering the line of longtime Avengers butler Jarvis. It's known as one of the fiercest battles the Avengers ever fought, and that's due to the assembled power levels they were confronted with. Still doesn't count.
Cinephile since 1993, aged 4, when he saw his very first film in the cinema - Jurassic Park - which is also evidence of damn fine parenting. World champion at Six Degrees of Separation. Lender of DVDs to cheap mates. Connoisseur of Marvel Comics and its Cinematic Universe.