Galactus Vs. ANYONE IN THE MARVEL OR DC UNIVERSE
GALACTUS WINS.
The only time Galactus "died", he was reborn out of the heart of Eternity. He can't lose. He existed BEFORE the current universe even existed, and it's been said numerous times that he/it is an integral part of the fabric of the universe. Even Odin said he was a badass.
So, to recap...
Galactus vs. Magneto. Galactus wins.
Galactus vs. Thanos. Galactus wins.
Galactus vs. The JLA, X-Men, Avengers, Alpha Flight, JLI, Sinister Six, Squadron Supreme, Superman, WildC.A.T.S., Wetworks, Gen13, and any other top teir team you can come up with off the top of your head. Galactus wins.
The simple fact that Galactus can CREATE beings powerful enough to wipe out the Avengers without working up a sweat should be some indication of his limitless power. Read that again - LIMITLESS power. The Watcher said himself that Galactus was not his equal, but his better. That's the Watcher, for Heaven's sake. So, barring what some writers have done to "weaken" Galactus (because having a truly omnipotent being makes for very uninteresting stories), Galactus is the single most powerful being in continuity. Forget the Phoenix, writers keep cramming the Phoenix down our throat to keep Jean, Scott, and the Summers line interesting. The Phoenix is a powerful entity - but it is a product of THIS universe. Galactus - as I've stated - is a product of another universe entirely, and he is the physical manifestation not only of that universe's brightest mind, but the entire sum total of its energies. In Excalibur #25, the Phoenix "defeats" a "malnourished" Galactus, but in earlier comics it was stated that if he needed to, Galactus could call upon the entire energy of the universe if needed. This seems like a writing trick based solely on character popularity - since every other being in the Marvel universe seems to understand that Galactus is the supreme power, and hardly anyone even thinks to mention Pheonix in the same breath as him. Odin himself called Galactus a force of nature.
For some insight into how the Marvel license-holders look at Galactus - take this for what it's worth - look at the game Versus. When Upper Deck asked Marvel who should have a raid deck (signifying beings so powerful that they couldn't be given a regular character card) the unanimous vote was for Galactus.
So, I hope this puts to rest all the bickering back and forth about who would win. Any time you see "Galactus vs..." just ignore the rest of it and know that Galactus would win. Easily.