Sliding Scale of Villain Effectiveness

An antagonist can be classed on three orthogonal parameters:

How much danger they, or their plans, pose.
How effective they are.
How much the audience is supposed to hate them.

This is a method of quantifying the second one.

Note that this does not cover any degree of sympathy; rather, it covers how probable their success is. To expand on this scale's orthogonality to the other two: it is entirely possible for a Complete Monster to fall at 'credible' or even 'low' on this scale (for example, a monster who is merely chief henchman to, say, Ming the Merciless); and it is possible to have a villain who bats at the level of Superman, but is so dumb that most of his threat comes from the fact that his plans will inevitably fail in a spectacular way.

Examples: Most Looney Tunes villains, Dr. Drakken from Kim Possible, Invader Zim (most of the time), Waluigi, The Ice King, The Dark Hand, many villains written by Dean Koontz, Dr. Doofenshmirtz, the Diamond Dogs, Mashmyre Cello (in the first half of the series), Dr. Eggman (from Sonic Adventure to Sonic Unleashed), The Pack. The Dark Kingdom qualifies due to Queen Beryl's poor management, though most of its villains are Credible threats in their own right.

Examples: Most Disney villains, the Master from Doctor Who, Invader Zim (on his worst days), Ming the Merciless from Flash Gordon, The Galactic Empire from Star Wars, The Decepticons (occasionally highly effective), Twilight's Hammer, The Fire Nation (whenever Azula isn't available as a strategic consultant), Tarrlok and Verrick, The Enclave, Sauron during the late Third Age, Dr. Eggman (pre-Sonic Adventure and post-Sonic Unleashed). Bowser (pre-Super Mario Sunshine) fits here, but is frequently highly effective against anybody other than the Super Mario Brothers, though he'd be more effective if he didn't kidnap Princess Peach. The typical villains from Power Rangers / Super Sentai also fall into this category, Most Redwall Villains, Schlemi/Fifi, Brizon and most minor villains, Demona, The Death Busters and Dead Moon Circus, The Principality of Zeon under the Zabis, The Titans, Patrick Zala & Muruta Azrael, Lord Djibril, The Homunculi, The Covenant.

Examples: The Daleks, David Xanatos, The Light, Gendo Ikari, Kefka Palazzo, Littlefinger, Hazama, Yuuki Terumi, Relius Clover, and Izanami, Palpatine, Darth Vader, Grand Admiral Thrawn, Princess Azula, Amon, Vaatu, Unalaq, Zaheer and Kuvira, Freeza, Cell, and Majin Buu, The Black Moon Clan, Sailor Galaxia and Chaos, Katsuhiko Jinnai, Richard Wong, XANA, Shego, Ra's al Ghul, the Joker, and Bane, Bowser (The Galaxy games and pre-Super Mario Sunshine) whenever the Super Mario Brothers aren't around, ^note Lionel Luthor and Brainiac, Rau Le Creuset, Haman Khan, Morgoth (pre-War of Wrath) and Sauron (pre-Third Age), Thanos, The Lich, Raul Menendez, Naraku, Kyubey, Nightmare Moon, Discord, Queen Chrysalis, King Sombra, Lord Tirek, Starlight Glimmer and Cozy Glow, Dio Brando, Kars, Yoshikage Kira, Diavolo, Enrico Pucci, Funny Valentine, The Indominus Rex, President Snow, Shogo Makishima, Kirito Kamui & the Sybil System, His Divine Shadow, Mantrid, Prince and Vlad, Malcolm Merlyn/Dark Archer, Slade Wilson/Deathstroke, Ra's al Ghul and Damien Dhark, The Major, Katz, Azazel, Lucifer and Crowley, Master Xehanort, Goku Black, Zamasu, Tai Lung, Lord Shen, Cinder Fall, The Gravemind, Galeem and Dharkon.

Infinite: The villain will always win, and the heroes will always lose, even if the heroes should by all rights win. This may be due to the villain being truly all powerful, or the heroes being utterly incapable of confronting them in the first place, let alone stopping them. At best, the heroes could delay or annoy the villain, but the villain will triumph in the end regardless and the heroes can only evade or escape the villain once its goals are met.