With 18 (outsider) HD and +2LA, a Pit Fiend is supposed to be the equivalent of a 20th-level character.
Given its yuuuge stat bonuses, excellent natural attacks, and useful SLAs, how does it stack up against other 20th level characters?

20th level Warblade? The Warblade might one-shot it as an ubercharger, but will have to bypass invisibility, DR, and 100' telepathy (seeing the warblade coming) to do so. If they start adjacent to each other, the Warblade might win. The Pit Fiend can probably kill it in 2 rounds (average damage output from the natural attacks is around 90hp). Possibly less when you factor in power attack, plus fireball damage, plus a chance of poison or grapple, plus a summon, plus being able to Power Word: Stun on round 2 with no save once the Warblade is below 150hp.

20th level Warlock? Pit fiend has more damage options and is tough enough to power through most crowd control. Pit fiend win.

20th level Wizard/Sorc? If the Pit Fiend can get physical, it wins; if the arcanist doesn't have enough HP, the pit fiend auto-wins with PW:Stun. With standard 20th-level items, the Pit Fiend will save against most wizard spells (+5 resistance, +1 luckstone, +3 to saves from Belt of Magnificence = saves of +28/+28/+30 vs a 30-int caster's DC of 29), and it has a nice package of immunities including SR that will block about 20% of the spells from a 20th-level caster with Spell Pen feats. A caster will have to do some serious pre-planning to defeat a Pit Fiend, whereas a Pit Fiend requires no pre-planning to stand a better-than-50/50 chance against the arcanist... unless we're getting into clones, demiplanes, astral projections, and other TO stuff that I've never seen anyone actually use.

20th level Druid? I assume the Druid can physically match the Pit Fiend, but the Pit Fiend has better offensive spells, and the druid will have a hard time beating the DR with natural weapons. I have never seen a druid take Spell Pen, so SR will negate a higher portion of the druid's spells. The Pit Fiend will probably have better saves as well.

I believe that a ECL 20 pit fiend as listed on the SRD is substantially superior to almost any build of 20 class levels unless the build is very, very highly optimized. I would put this at LA+4 or LA+5 so that other members of the party have a chance to at least pick up a couple of Epic feats to make up for some of the major disadvantages they face vs the Pit Fiend chassis.