Who is the strongest superhero or supervillain in Marvel or DC comics?

I'll leave it to the comic geeks to answer but according to Marvel's
own site they talk about the Incredible Hulk as being: "the most
powerful man-like being of all time!" 
http://www.marvel.com/catalog/?id=3638

If you exclude the part about "Marvel or DC comics", I would say it
would be Howard Stern's "Fartman" character.

There's probably no definitive answer to this as there are comic
characters whose powers are virtually unimaginable.

For examples, check out:

http://www.marveldirectory.com/strengthlevels.htm

For instance, take Marvel's Galactus who

"possesses cosmic power beyond the abilities of human beings to
measure. Galactus is able to use the vast energies within him for an
incalculable number of effects. Some of his most rudimentary abilities
are the molecular restructuring or conversion of matter, the
projection of energy with enormous concussive force, the teleportation
of objects across space or even time, and the erection of invincible
energy-screens and force fields. Even his heralds, whom he has endowed
with a small fraction of his ever-renewing power, are able to
manipulate matter and energy in ways far beyond human comprehension."

Kind of makes Spiderman look like a bit of a wuss...

Have a good day.

i think it's venom.

Bane is the strongest supervillan of the DC universe.

In DC, it is probably Darkseid.  Or Pre-Crisis Superman.

SUPERMAN!  He can turn back time by forcing the earth to spin
backwards, now That's Strong!

Though ironic that this character died, Beyonder is for me the most
powerful of all. I guess that's why the Marvel editors then had him
eliminated.

http://en.wikipedia.org/wiki/Beyonder

In terms of actual comic characters, that honor would go to One Above
all and the Presence. Out of omnipotent beings, they're the MOST
omnipotent, as little as that makes sense.

In terms of people who actually fight crime rather than being abstract
beings and the like, Superman. It's been stated quite a few times
(especially by John Byrne, a comic book writer) that Superman has near
limitless strength and power which he almost never taps into.
Sometimes he might be beaten by someone, but it's been shown that he's
simply afraid to ever realy cut loose with his ful powers and as he's
seen by many writers as a Jesus/God analogy they're written into the
works that if he ever truely did cut loose he'd be unbeatable, which
was especially apparent in his joining with kismet to fight Imperiex
during Our World at War where it was stated that for a split second he
was virtually limitless.

Also the Beyonder is only an incomplete cosmic cube, and all of his
biggest showings of powers were actually illusions. He's pretty
powerful, but by no means the most powerful