Galactus
survived the end of the last universe, eats planets, made the human alien into the invincible Surfer after Norin bargained with the big man to save his planet. In return Norin helped Galactus snack on other worlds.
Silver Surfer's power cosmic allows him to travel FTL across the Galaxy, surive a supernova to the head, or atomically rearrange matter such a changing oxygen molescules to gold or whatever, Surfer has total control over the laws of physics
As power as Surfer is, his powers are totally dwarfed by Galactus
Thanos
Overpowered evil villain on the level of classic Darkseid, he has most of Surfer's cosmic powers, murders Superman level guys with just palm slaps, he has an intellect that puts Batman to shame. He is obsessed with technology, dark arts, mysticism and is bared from deaths realm leaving him totally immortal.
Imperial fleets
Skrulls, Shiar, Enslavers, Kree all those kinds of guys with big Empires and big spaceship and superpowered foot soliders
Old Earth Gods
Norse Gods, Greek Gods, Celtic Gods Akhenaten, Loki those kinda guys. Since lots of these guys are immortal and magic doesn't really obey the laws of physics these guys often give Surfer trouble
Dr. Doom
The one Earthling that gives him real problems, Doom is the geordi/o'brien/data of the Marvel verse

but he uses his powers for evil and is a dictator in Eastern Europe

. Doom has often invented devices that knocked Surfer on his butt
Elders
These cosmic beings have been around since the start of the universe. You have guys like Champion (who beats everyone down with his strength feats) or guys like Gardener (an immortal guy who is obessed with growing plant life)
Demons
Mephisto (Marvel's version of the devil), his son Blackheart and Elder Demons
Impossible Man
A guy with "Q" or Mxy like powers who can make himself into any shape imaginable.
Crossovers
Surfer has fought Orion, the Green Lantern Kyle, Cyborg Superman and Mr. Mxyzptlk in cross overs
Other Heralds
Heralds are often given powers very similar to Surfer and not all of them came from socities as advanced or peaceful as Norins. Some of them came from war torn worlds, others still serve Galactus in helping him find worlds. Ever since Surfer became free he frequently fell into conflict with other Galactus heralds. As powerful as they may seem on paper not all of them are so tough many of them are like wrestling jabronis. The only two who really gave Surfer trouble would be the herald known as Morg and the herald known as RedShift (a guy who once took on Surfer whil fighting inside a blackhole). Other heralds haven't been too much trouble for Surfer, Firelord maybe could go a few rounds with him
Terrax, no this guy is getting his butt beatdown
Others....
Once you get into Marvel guys like Hulk, Ironman, Wolverine, Thing etc
You start counting the number of times Surfer has beat these guys down and let them live without them being aerosolized