A personal top ten list of the absolute worst abilities, summarized.
Absolute worst
10. Heavy Metal: This ability is more of a hindrance than anything, as most of the users are weak to low kick or grass knot. And powering up heavy slam isn't very helpful anyway, since steel typing is very poor coverage, and there are better abilities for aggron.
9. Run Away: Out of all ingame non-competitive abilities, this is the only one that failed. Battle Pyramid proves this. I try to escape with a Run Away pokemon: Can't Escape. Dead. Battle pyramid's wild pokemon has like an invisible quick claw; has status moves, and can prevent you from escaping, even with your fastest pokemon. Run away wouldn't be on this list if it did what it was supposed to do, but it failed miserably as an ability, which is complete ********.
8. Normalize: An ability that completely removes type coverage. Ghost types, especially with shadow tag, will screw you over. And delcatty, a pretty terrible pokemon, gets this ability. The ONLY gimmick this has is making ground types affected by thunder wave. And I heard it got slightly better in future gens.
7. Weak Armor: Another ability that fails. It requires a turn to work, and agility, dragon dance, quiver dance, automize, and most glaringly Shell Smash, makes this ability look really dumb.
6. Stall: One of the rarest, and most forgettable filler abilities. It really feels tacked on, and can only work with payback... but it makes any attack have negative priority, and is used by only one relatively weak pokemon. Think of the ability as lagging tail.
5. Tangled Feet: What could've been a troll ability fails as one. For it to work, you have to be confused. So at best, the 20% evasion lasts 5 turns max. At worst, 1 turn. And if you hurt yourself, and the enemy attacks, 80% chance of being hit. Very poor tradeoff. Sand Veil and Snow cloak, or even brightpowder is better.
4. Defeatist: The first of the crippling abilities. However, out of all of them, it is the least bad, since it doesn't take effect immediately. But on archeops, which is outclassed by aerodactyl, who doesn't have this ability, deserves a low spot on the list.
3. Truant: A crippling ability worst than defeatist, since it is somewhat immediate, but not as bad as the later ones. Slaking is still usable, with choice band and prediction, and entrainment durant can troll the battle simulators. But I would take defeatist slaking over truant anyday. Substitute users or protect users will destroy poor slaking. And poor slakoth is stuck with this horrendous ability, and its 1st stage stats are still crappy compared to many other 1st stage pokemon.
2. Slow Start: This stupid ability cripples the pokemon immediately, and lasts until five turns. Against any competent player, the victim won't last five turns. Worst is that its on a Legendary, made worse by the fact that there is are 2 legendaries with base 660+ stats without the crippling ability, that are still not considered overpowered. Slow start makes Regigiggas unusable for sweeping, and can only play support, which is outclassed by non-legendaries. Slaking and Archeops would much rather prefer their own terrible abilities, rather than slow start. Regigiggas the worst legendary in the series, putting it on the same tier as luvdisc.
1. ZEN MODE: Surprised that there is an ability that's worse than slow start? This ability failed because in order for it to work, you need to have darmanitan at less than 50%. Because of the 50% decrease, any bulk from zen mode darmanitan is erased, and it loses its speed. This pokemon is completely unusable because of the requirements for this ability to work. Even slow start wouldn't hurt the blue darmanitan considering high special attack stats, low speed, great bulk. What's worse is that this ability could've been cool like imposter or illusion, but instead of working immediately, it requires darmanitan to have less than 50% hp. This is far worse than defeatist, since it does a similar thing, only instead of halving the physical attack and special attack, it quarters its attack and speed down to nothing, and gives useless defenses due to the low HP. Consider this, Archeops, Slaking, and Regiggigas have competitive movesets. Zen Mode Darmanitan does not. Thats how worthless Zen Mode is.
Some mentions:
Klutz is useful for lopunny, for tricking choice items, toxic orbs, iron balls, lagging tails, etc. And believe me, its really annoying to be tricked a choice item.
Illuminate can be useful in places like the safari zone, or places with rare encounters. So the ability isn't useless. Unlike Run Away, where the one place it could've worked, didn't work.