The original Wonder Man (not Marvel, theirs came later on and the name was free so they took it) was so blatantly a Superman ripoff, the publishers were ordered to stop making his comics. This was very early on so the laws regarding public fictional characters where a bit different. A similar thing happened with Captain Marvel, but they only had to pay a massive sum to DC, leaving them nigh bankrupt, so afterwards they sold all their assetts to DC.
Squadron Supreme anyone? Although there's actually a good reason for them. Back when they debuted, the writers on JLA and Avengers wanted to a crossover. Too much redtape and too many editors saying "no" kept them from it. So the writers basically made the ripoff teams of the others and told the story they wanted to. We got Squadron Supreme, with a much longer lasting legacy, and later on they told the FIRST "Superheroes take over and run the world" story. Though it's been redone, and done better since then, it was a FIRST.
In JLA the same month, roughly, as when the Squadron debuted, we saw a few suspicioulsy familiar characters appearing in JLA, they were familiar as they were Avengers knockoffs! They've appeared here and there since then, but didn't have the staying power of Squadron Supreme.
Prime from Malibu is a SHAZAM ripoff, I think. Although, in place of pantheon and wizard, we get covert military stuff. They're both kids who transform into powerful bodies capable of flight, awesome feats of superstrength etc.
Aquaman is a Namor ripoff. Dual species heritage, king of Atlantis etc. Although Namor has the footwings and superstrength.
Red Raven is a Hawkman ripoff, a human adopted by a species of Bird People after a plane crash, and given artificial wings capable of endowing him with flight. He's also been seen with a mace....hmmm.
Wizzer was a Flash ripoff, quite blatantly. They wanted to cash in on the speedster archetype.
Thin Man was actually the first stretching hero, predating Plastic Man by about a year. So people saying Mr. Fantastic is a Plastic Man ripoff are actually wrong, it was just Stan recycling ideas from earlier Marvel heroes.
Patriot, Defender and others were Cap ripoffs, though all made by the same company.
Though not all that similar, the original Vision, Aarkus was an extradimensional alien, with a green countenance. He could fly and do a host of stuff depending on the story. There are some Martian Manhunter parallels.
That's all I have off the top of my head.