Since apparently reposting locked threads is good now...
Foreward
This system is to help you balance your campaign so that everyone is able to face roughly the same power in challenges (basically, so nobody is too powerful or not powerful enough and they can all contribute in encounters). This is *not* a system about breaking the game or exploits (because exploits break the game too). I am defining breaking the game as one of too things:
1) Making appropriate encounters trivial (examples include explosive runes bombs, shrink item bombs, wish economy without book of gears, basically anything that gets arbritrary really)
2) Things that make the PC unable to play the same game. An example of this is the monk. There is literally nothing you can do to make him play the same game as a beguiler.
Other things this is not about:
High level optimization. If you're actually needing my help to balance your campaign with a tier system, you straight up don't know enough to make top end optimized characters. I am going to assume moderate competence (such as being able to figure out which spells in the PHB are really awesome on wizards).
Tier 1: Always has a top end level appropriate effect and pretty darn good defences.
Examples: Wizard, Cleric, Druid, Archivist, Artificer.
Tier 2: Always has something to contribute that's totally fricken awesome, but might not be as good as the tier 1 guys.
Examples: Favoured Soul, Psion, Sorcerer, Dread Necromancer, Beguiler, Wu Jen, Spirit Shaman
Tier 3: Specialists that are good in a specialty that is straight up inferior (for example, melee guys compared to SoD casters above) and caster types that don't get as good spells as the tier 2s (be it because of a gimped spell list like shugenja or crappy power learning mechanic like ardent).
Examples: Crusader, Rogue, Binder, Duskblade, Warblade, Psychic Warrior, Incarnate, Totemist, Shugenja, Ardent
Tier 4: Competent in area of specialty, can probably get 4/10 wins on the same game test. Typically one trick ponies.
Examples: Swordsage, Barbarian, Warlock, Warmage, Scout, Ranger, Adept, Fighter, Bard, Factotum, Rokugan Ninja
Tier 5: I cut myself at night classes. These classes have trouble even in the area of specialization.
Examples: Monk, CA Ninja, Healer, Swashbuckler, Soulknife, Expert, Paladin, Spellthief, Hexblade, Marshal, Soulborn
Tier 6: I just shot myself in the balls classes. These classes can't do anything even if they specialize in it.
Examples: CW Samurai, Aristocrat, Warrior, Commoner, Lurk, Divine Mind
Tiers 1 and 2 are generally fine together. 3 and 4 are also pretty ok together. I wouldn't suggest letting your players pick tier 5 or 6 classes in any published adventure.