advanced pit fiend
Several folks in this thread have mentioned how the 3.0 pit fiend needed a boost in power to be the big terror they are billed as. Well, remember that the monsters listed in the MM are considered the smallest, weakest, and most common of the given species. So to make the more impressive version, instead of changing the base pit fiend stats, you could just use the advancement rules for 3.0. Here is an advanced pit fiend. (Note this is not even the most advanced possible.)
Devil, Pit Fiend
Huge Outsider (Baatezu, Evil, Extraplanar, Lawful)
Hit Dice: 26d8+182 (299 hp)
Initiative: +4 (+4 Improved Initiative)
Speed: 40 ft., fly 60 ft. (average)
AC: 31 (-2 size, +23 natural) touch 8, flat footed 21
Base Attack/Grapple: +26/+44
Attack: Claw +35 melee (1d8+11) Full Attack: 2 claws +35 melee (1d8+11) and 2 wings +33 melee (1d6+5) and bite +33 melee (2d8+5 and poison plus disease) and tail slap +33 (2d6+5)
Face/Reach: 10 ft./15 ft.
Special Attacks: Constrict 2d6+16, fear aura, improved grab, spell-like abilities, summon baatezu.
Special Qualities: Acid resistance 20, cold resistance 20, DR 25/+2, fire immunity, poison immunity, regeneration 5, see in darkness, SR 28, telepathy.
Saves: Fort +22, Ref +15, Will +20.
Abilities: Str 33, Dex 11, Con 25, Int 20, Wis 20, Cha 16.
Skills: /snip - didn't have time to go through all these, but it would have 338 skill points to work with
Feats: Cleave, Expertise, Great Cleave, Improved Disarm, Improved Imitative, Iron Will, Multi-Attack, Power Attack, Quicken Spell-like Ability.
Tactics Round-by-Round
The pit fiend opens with spell-like powers, attempting to neutralize dangerous opponents before entering melee combat.
Round 0: Unholy aura; activate fear aura; quickened summon Baatezu.
Round 1: Hold person against unarmored opponent (preferably a spellcaster).
Round 2: Meteor swarm against as many foes as possible, approach worst-injured enemy.
Round 3: Full attack against injured enemy.
Round 4: Continue melee against injured enemy, or hold person against annoying spellcaster.
Round 5: Repeat from round 1; or greater teleport to safety if endangered.
The above pit fiend stats are using the 3.0 rules, but with the 3.5 layout. I still hold that the base stats didn't need changing. To make a more powerful pit fiend, a DM only needs to spend a few minutes (as I just did here) to advance the creature, or maybe add class levels to it. Imagine a standard pit fiend with a dozen fighter or sorcerer levels!
Quasqueton