Best singular class abilities, features, etc.?

[noradetzky]

You know, because an illithid savant needs to know what s/he's going to attack.

[Particle_Man]

The ability to cast spells would be up there for clerics, druids, wizards, sorcerers and the like.

The Factotum ability to imitate another class's (ex) ability would be noteworthy too.

[Eonir]

Wild Shape. End of story.

[Kuulvheysoon]

Loyal Beyond Death (Knight 20).

Hands down.

[Metahuman1]

Love me some Spirit Lion Totem, Trap Killer, and Stance Mastery.

[Hiro Protagonest]

Loyal Beyond Death (Knight 20).

Hands down.

...No, not really. Seriously, it does give you an extra defense against SoDs if you have some sort of healing ability, but in most cases, Rageclaws with a couple points of essentia invested is better.

I'm not sure what you're asking? Best single-classed build, or best class feature? If class, straight wizard, hands down, maybe straight druid in core-only. If class feature, Wild Shape.

[HunterOfJello]

Wild Shape

Plant Shape from Landforged Walker [SoX]= Wildshape into a plant creature (size small to huge) 3/day using your total character level as your effective druid level.

Aptitude Focus (Ex) from Chameleon [RoD]= Ability to cast level 1 through 6 cleric or wizard spells without the limit of 1 spell per level. (Note: It is explicitly stated that Aptitude Focus is NOT spellcasting. It is an Ex ability. Therefore, it would be open to be gained as a class ability without the spellcasting restrictions.)

Aptitude Focus (Ex) + Double Aptitude from Chameleon [RoD]= both Cleric AND wizard spells from levels 1 through 6

Bonus Feat (Ex) from Chameleon = 1 chosen bonus feat of your choice every day which you can switch every day

Immortality OR Spell Resistance from Dragon Ascendant [Draconomicon] = Become a quasi-deity and immortal or gain Spell Resistance 33 as a class feature

~~~~~~~~~~

Becoming a quasi-deity should give you Divine Rank 0 and access to all sorts of other shenanigans.

Also, http://www.giantitp.com/forums/showthread.php?t=186625&page=2 might have some other good ideas.

[Biffoniacus_Furiou]

Initiate of the Sevenfold Veil (CA) gets a single Warding (Sp) ability to which each individual veil is added as he progresses. You would have to gain Reactive Warding and other abilities separately, but Warding alone gives you all seven colors if taken from a high enough level initiate.

A Choker (http://www.d20srd.org/srd/monsters/choker.htm)'s Quickness should be at the top of your list, right along with the Tarrasque (http://www.d20srd.org/srd/monsters/tarrasque.htm)'s Regeneration ability, which includes its only-killed-by-wish clause. Get a Will-O'-Wisp (http://www.d20srd.org/srd/monsters/willOWisp.htm)'s Immunity to Magic and you're outright immune to that Wish or Miracle unless it's being cast by a Dweomerkeeper (http://www.wizards.com/default.asp?x=dnd/we/20040522a) using Supernatural Spell.

I actually played an Illithid Savant a while back in an epic game. I wasn't able to get the Tarrasque's Regeneration, but I was able to consume other Illithid Savants and spending a single Acquire Class Feature to gain three Acquire Class Features, which I then used to acquire additional special attacks/qualities and yet more class features. For offense I used the following acquired abilities: Supreme Power Attack (Frenzied Berserker, CW), Powerful Slams (Goristro, FC1), Shadow Pounce (Telflammar Shadowlord, UE), Dimension Door (Blink Dog, MM), Spellcasting (Sorcerer 20, PHB), and Quickness (Choker, MM). I also legitimately gained Pounce (Barbarian 1, Lion Spirit, CC), Leap Attack (feat, CV), Shock Trooper (feat, CW), Improved Rapidstrike (feat, Draconomicon), and I think Incantatrix 4+. I had a full array of Persistent spells (Wraithstrike, Ray Deflection, etc.) with Divine Power, I was only allowed to steal one class worth of spellcasting but I added it to his Sorcerer list by acquiring the feat Orc Descent in Dragon 338. I also had Automatic Quicken a few times, and Multispell a few times.

His two slam attacks were each considered two-handed weapons, and he got full iterative attacks without TWF penalties thanks to Improved Rapidstrike, which was actually 5+ attacks per fist per full attack since it doesn't cap at +16 BAB. He used a full-round action to charge with Shock Trooper, using Power Attack and Leap Attack to get +8 damage per -1 to AC, which was in the neighborhood of +200 damage. I also somehow had Valorous and Speed on them, probably via consuming a Kensai, and also Wounding and Marrowcrushing (BoVD) for 2 Con damage per hit. That's a charge for 12+ attacks at around 500 damage per hit, making touch attacks. I also may have had Arcane Duelist (http://www.wizards.com/default.asp?x=dnd/re/20030224a) 3+ to get an extra ~25 to hit for an equal penalty to damage. After that he would use his Blink Dog Dimension Door (free action) to Shadow Pounce, and gain another full attack at all the same bonuses. He would then used Quickened teleport effects (Benign Transposition with a familiar I think) to get additional Shadow Pounces. This was all after using his standard action via Quickness to MDJ the opponents, ensuring that none of them had any defensive buffs remaining. Needless to say, every encounter died as soon as I acted, and I used Moment of Prescience for initiative.