The greatest enemies of Ben Tennyson, a boy who got watch-like Omnitrix device with which he can transform into different alien forms and thus became a superhero.

#20 A humorous space criminal with a small size and large ego/armor.
#19 Corrupted, arrogant and egotistical former (rogue) Plumber and Max's old partner who can transform into Terroranchula Form because he was user of the Nemetrix (anti-Omnitrix).
#18 Galactic bounty hunter, Ben's "Boba Fett". Assassin who works with #1 and #15.
#17 The main member of Forever Knights, organization of human supremacist knights frequently fought by Ben.
#16 An evil and psychotic Cerebrocrustacean scientist who invented the Nemetrix. He's one of main villains of Ben10 Omniverse.
#15 Main right hand man of #1 who also works himself.
#14 Horror clown, classic ben enemy
#13 Cthulhu - Shuma-Gorath style powerhungry extra-dimensional demonic entity and leader of Lucubras.
#12 Master magician, Ben's classic foe later overshadowed by his niece #7.
#11 The time traveling villain, probably the evil future version of Ben, strongly resembles DC's Black Beetle/Marvel's Kang.
#10 Handsome, well-mannered energy vampire with a bad habit of turning young girls into zombies.
#9 An intergalactic hunter and mercenary with his dogs, obsessed with hunting alien species, especially Omnitrix bearer. Ben's own "Kraven".
#8 Kevin's evil monster form, one of the main villains of the original series.
#7 Poweful sorceress and niece of #12, obsessed with gain more power like many other Ben enemies. Sworn enemy of Gwen.
#6 Incomplete and mutated Galvanic Mechamorph, technopath created by Azmuth. One of main enemies of Ben10 Omniverse and Ben's own "Ultron".
#5 Anti-Ben 10. Azmuth's fromer assistant and bearer of the Ultimatrix (anti-Omnitrix). He is unaccepting of Ben's right to wear the Omnitrix and wants it for himself.
#4 A terrifying ghostly alien from inside the Omnitrix who can control people and is afraid of sunlight. One of my favorites because I like the idea that the threat comes from inside the device.
#3 The first and the most iconic and the most frequently occurring Ben's villain with #1. Typical mad scientist with animal control and gadgets. Ben's own "Sivana/Baxter Stockman/Spencer Smythe/Leader".
#2 An intergalactic criminal seeking ultimate power. One of Ben's main opponents.

#HM1 Leader of a heavily-armed female biker gang.
#HM2 cybernetically-enhanced crab-like bounty hunter mercenary.
#HM3 Cyborg-like space Frankenstein.
#HM4 Mutant criminal who can control insects.
#HM5 Intergalactic crime boss which even #1 fears.

#1 Intergalactic warlord, The big bad of the Ben universe. Ben's own "Thanos/Frieza".