After realizing how many methods there are of stacking attack penalties on enemies in this thread, I've decided to put together a guide to put them all in one place for handy reference. I hope you like tables. Also, there will be a stronger focus on at-will attack penalties, because that's more fun.
The beauty of these builds is that a single hit will greatly cripple any enemy without an automatic damage mechanism - if you have good defenses, monsters will only hit on an 10 or better, and if they have a -9 to attack, they're practically helpless bar critical hits. Plus, there's something so satisfying about imagining a rogue dancing around an enemy taunting him with "hit me, c'mon, hit me!".
Illusion Apprentice: Hit with illusion power, -2 to Next attack roll
Use club and mace for rogue powers, qualify for dizzying mace
Messenger of Peace
Aura of Peace (16): Enemies within 2 squares of you take a -2 penalty, All, Always
Shadow Assassin’s Riposte (11): When missed, deal enemy Dex damage
Transcendent Blessing (Oghma, 11): Singing Strike also immobilizes until EoYNT
Fearsome Zone (16): Enemies in zones of your psionic powers take a -2 penalty, All
Dizzying Mace may or may not work with Overwhelming Impact, as “using” a mace may or may not include having a mace in your off-hand. Some rods also count as maces, for adding Dizzying Mace on Warlock implement powers and so on.
I've been playing around with methods of stacking attack penalties on enemies at-will. Marks and mark-like abilities are good, but preventing hits on all targets is better - and general build practicality is still valued. Here are a few theoretical build frames, two for single target and the one for multiple targets.
Half-Elf Rogue/Paladin Versatile Master with Enfeebling Strike, Psychic Lock + Githyanki Silver Dagger, Multiclass Paladin with Power of Madness, Underhanded Tactics and Mark of Warding with Defending Dabbler [7 feats]
Enfeebling Strike: -2 (all) Mark: -3 (allies) Psychic Lock: -2 (next) Power of Madness: -1 (all) Underhanded Tactics: -2 (all) TOTAL: -7/5 against you, -10/8 against allies (the two values represent first and subsequent attack rolls, as Psychic Lock only hinders the next attack)
Rattling: -2 (all) Psychic Lock: -2 (next) Underhanded Tactics: -2 (all) Distracting Companion: -2 (you and companion) TOTAL: -8/6 against you, -6/4 against allies (the two values represent first and subsequent attack rolls, as Psychic Lock only hinders the next attack)
BONUS: Arena Specialist [Arena Fighting] means adjacent enemy is rattled as well
Half-Elf Rogue Versatile Master with Beguiling Strands, Psychic Lock, Underhanded Tactics and Mark of Warding with Defending Dabbler [5 feats]
Mark: -3 (allies) Psychic Lock: -2 (next) Beguiling Enchantment: -2 (you) Underhanded Tactics: -2 (all) [one target] TOTAL: -6/4/2 against you, -7/5/3 against allies (the two values represent first and subsequent attack rolls, as Psychic Lock only hinders the next attack, and the third represents the difference between all targets and one target you chose to hit with sneak attack)
Add Resounding Thunder + Arcane Admixture to make it close blast 6 and/or MC Fighter + Arcane Implement Proficiency (Glaive) + Polearm Momentum to add prone (extra -2 penalty)
Half-Elf Rogue Versatile Master with Eldritch Strike, Psychic Lock + [some means of psychic damage?], Hammer Shock, Underhanded Tactics and Mark of Warding with Defending Dabbler [6 feats]
Rattling: -2 (all) Mark: -3 (allies) Psychic Lock: -2 (next) Underhanded Tactics: -2 (all) TOTAL: -6/4 against you, -9/7 against allies (the two values represent first and subsequent attack rolls, as Psychic Lock only hinders the next attack)
*similar to enfeebler, but get to add all the benefits of MBA
Half-Elf Rogue Versatile Master with Vicious Mockery, Psychic Lock, Underhanded Tactics, Beguiling Strands and Mark of Warding with Defending Dabbler [6 feats]
Hit: -2 (all) Mark: -3 (allies) Psychic Lock: -2 (next) Beguiling Enchantment: -2 (you) Underhanded Tactics: -2 (all) TOTAL: -8/6 against you, -9/7 against allies (the two values represent first and subsequent attack rolls, as Psychic Lock only hinders the next attack)
*like the beguiler, but single target in exchange for more reliable penalty
Here is the section for penalties that occur daily, per encounter or action point. If you have enough of these, you can easily stack unbeatable penalties for most of each fight - see the sample builds for examples.
F-111 Interdictor Long (200+ squares) distance ally teleporter. With some warlord stuff. Broken in a plot way, not a power way. Thought Switch Higher level build that grants upto 14 attacks on turn 1. If your allies play along, it's broken. Elven Critters Crit op with crit generation. 5 of these will end anything. Broken. King Fisher Does an excellent job at keeping an enemy disabled in a few ways. Strong. Boominator Fun catch-22 booming blade build with either strong or completely broken damage depending on your reading. Very Distracting Warlock Lot's of dazing and major penalties to hit. Overpowered. Pocket Protector Pixie Stealth Knight. Maximizing the defender's aura by being in an ally's/enemy's square. Yakuza NinjIntimiAdin: Perma-stealth Striker that offers a little protection for ally's, and can intimidate bloodied enemies. Very Strong. Chargeburgler with cheese Ranged attacks at the end of a charge along with perma-stealth. Solid, could be overpowered if tweaked. Void Defender Defends giving a penalty to hit anyone but him, then removing himself from play. Can get somewhat broken in epic. Scry and Die Attacking from around corners, while staying hidden. Moderate to broken, depending on the situation. Skimisher Fly in, attack, and fly away. Also prevents enemies from coming close. Moderate to Broken depending on the enemy, but shouldn't make the game un-fun, as the rest of your team is at risk, and you have enough weaknesses. Indestructible Simply won't die, even if you sleep though combat. Sir Robin (Bravely Charge Away) He automatically slows and pushes an enemy (5 squares), while charging away. Hard to rate it's power level, since it's terrain dependent. Death's Gatekeeper A fun twist on a healic, making your party "unkillable". Overpowered to Broken, but shouldn't actually make the game un-fun, just TPK proof. Death's Gatekeeper mk2, (Stealth Edition) Make your party "unkillable", and you hidden, while doing solid damage. Stronger then the above, but also easier for a DM to shut down. Broken, until your DM get's enough of it. Domination and Death Dominate everything then kill them quickly. Only works @ 30, but is broken multiple ways. Battlemind Mc Prone-Daze Protecting your allies by keeping enemies away. Quite powerful. The Retaliator Getting hit deals more damage to the enemy then you receive yourself, and you can take plenty of hits. Heavy item dependency, Broken. Dead Kobold Transit Teleports 98 squares a turn, and can bring someone along for the ride. Not fully built, so i can't judge the power Psilent Guardian Protect your allies, while being invisible. Overpowered, possibly broken Unnamed Avenger|Runepriest/Hammer of Vengance Do lot's of damage while boosting your teams. Strong to slightly overpowered. Charedent BarrageA charging ardent. Fine in a normal team, overpowered if there are 2 together, and easily broken in teams of 5. Super Knight A tough, sticky, high damage knight. Strong. Super Duper Knight Basically the same as super knight, only far more broken. Mora, the unkillable avenger Solid damage, while being neigh indestuctable. Overpowered, but not broken. Swordburst Maximus At-Will Close Burst 3 that slide and prones. Protects allies with off actions. Strong, possibly over powered with the right party.
Arcane Familiar - Gallant Hawk While a creature marked by you is adjacent to you, it takes a -3 penalty to attack rolls on attacks that don’t include you as a target instead of the normal -2 from being marked.
I am grabbing this one for my Paladin MC Bard since Mark of Warding isn't available.