Honestly, the best way to power down 3.X is to not use the PHB (outside of core game functions like skills). Only using books from 2004-2007 is much more balanced than PHB games.
PH core only druid is really strong. Wildshape with large dire bear at 8th level including improved grab claws with +23 grapple check is strong melee and BBEG or most any humanoid control. Can dump stat things like strength and still be a melee powerhouse. Plus animal companion and summon animal action economy masters. Plus full caster. Main weaknesses are things with DR (which can still be grapple pinned to lock down) and the first couple levels.
In my high level group the single classed archer focused fighter was a high AC machine gun of arrows that did more damage than anybody else.
Batman Swiss army knife utility wizard comes into play around mid level from core PH and stays strong throughout the game. It is also a SAD class so getting tough saves on save or die stuff is built into core. Those can be situational (mind affecting immune creatures, necromancy immune things, etc.) but they are really strong against a lot and they have other options. Their utility is huge, enabling safe resting, scry buff teleport, invisibility and knock. They can do buffs, information, direct attack, combat control, etc.
Rogue is among the weakest classes, core or with supplements, but arcane trickster prestige class is a competitive power class and it is from core DMG. Down a little on sneak attack and caster, but full advancing in both means they will generally hit with touch attack sneak attacks and do really good damage on the ray plus sneak attack spell attacks and improved invis enables lots of sneak attacks.
Cleric is strong, enough time to buff they can out melee the warrior classes. With core cure light wound wands they do not have to be primarily healers with their spells, but can use their spells for other and more direct effects.
Supplements starting with Sword & Fist made a ton of options, but usually big tradeoffs. Very few things were just stronger than core options. Most caster prestige classes either gave up a caster level which was huge or gave very minor extra benefits.
Ur Priest prestige class accelerated spell levels could be exploited, libris mortis undead creation feats, the kobold pun pun exploit of the Forgotten Realms Serpent Empires Sarrukh racial scaled folk genetic manipulation power. Psionic thrall herding could be an issue.
Most expansion things were just not up to the power level of single classed top tiered core stuff.
Tome of Nine Blades super power ranger classes gave those warriors mechanically interesting powers that were decent strength, but not core CoDzilla levels.
Beguiler was a really fun class with lots of interesting stuff flavorwise and mechanically, but it was not to the power level of a core wizard who could be an enchanter specialist but still have options against the many creatures immune to mind affecting spells.
Hexblades were full of flavor but were like light armored rangers with no healing, no animal companions, and debuffs instead of ranger spells.
PH2 druid alt unlimited wildshape from first level was flavorful and awesome, but required using the druid's human stats making them more MAD and not nearly as powerful as turning into 31 strength dire bear at 8th level.
