Most Powerful One-Level Dips

[Empirate]

Nobody mentions Conjurer? That Abrupt Jaunt is one of the better defensive abilities in the game, at any level. You get some low-level spells as well which might be just what you need on an otherwise warrior-like build (True Strike? Grease?), but you can also use scrolls, wands and staffs with Wizard spells now! Conjurer is a gooood 1-level dip.

[sirpercival]

But... conjurer is a good 20 level dip.  So I'm not sure that counts.  It's not front-loaded at all.

[Ryu Hayabusa]

I've got to second or third Marshal. A one level dip on charismatic characters solves so many problems for the party and initiative. I'd also like to suggest Psychic Warrior. For the cost of 1 point of bab, a two level dip grants you a pair of useful powers. That's much better than just dipping straight fighter if you need fighter feats.

[InnaBinder]

It's dubious because of entrance requirements, but Shadowdancer makes a pretty solid, popular 1-level dip for HiPS.  I could even see justifying a 2-level dip for a build that needed Evasion, qualified otherwise, but didn't have room for Rogue 2 or other Evasion-granting methods, though that's probably corner-case.

EDIT: The number of weapon proficiencies Shadowdancer grants can also be a boon to certain specific builds, I suppose.

[caelic]

I personally sort of like Ruathar as a dip; it can nicely diversify a caster's skillset.  However, I don't think it really qualifies for "most powerful."

[Bauglir]

In certain skill-based builds, Warlock is a solid dip, and moreso the lower your level. An unnamed +6 all day is pretty handy, although you might as well take two levels for detect magic and a second dip. Eldritch blast will almost never be useful in a dip, but there are probably niches, particularly if you don't particularly want to bother with buying a backup ranged weapon.

[Mushroom]

Paladin of tyranny is quite nice for a 2-3 level dip

[Endarire]

Contemplative (Complete Divine) gets you +1 domain.  Also, if you aren't a Cleric, you get Cleric casting.  (See the 'Spellcasting' section of this class.)

Hathran (Player's Guide to Faerun), with an acorn of far travel, gets you full spontaneous casting!

Swordsage, Warblade, and Crusader each give a stance and a buncha maneuvers.  Swordsage also gives Weapon Focus.  Warblade also gives Weapon Aptitude.

[ChupacabraJohn]

Both Incarnate and Totemist provide powerful 1-2 level dips, albeit the 2nd level is where the real power comes in.

Also (Dragon Fire) Bard 1 provides some awesome stuff when coupled with some feats.

[kurashu]

It may not be the super best, but Human Paragon is a nice little dip for 1, 2 and 3.

[phaedrusxy]

Nobody mentions Conjurer? That Abrupt Jaunt is one of the better defensive abilities in the game, at any level. You get some low-level spells as well which might be just what you need on an otherwise warrior-like build (True Strike? Grease?), but you can also use scrolls, wands and staffs with Wizard spells now! Conjurer is a gooood 1-level dip.

This makes me think: Erudite 1 is an awesome dip, also. I've used it in several ardent and psi-warrior builds to get extra feats and add access to a few 1st level utility powers.

[Nytemare3701]

But... conjurer is a good 20 level dip.  So I'm not sure that counts.  It's not front-loaded at all.

I think his point is that a single level in conjurer gives a wide ranged of utility compared to a single level in many other classes. Yeah, the wizard does most of it, but the conjurer is the best choice for most builds that want a single level for utility.

[TC X0 Lt 0X]

Ardent 1 is not a bad one.
With Practiced Manifester, you can have access to 3rd Level Powers (although you would be lacking in the PP department).
Most of the mantles are not as powerful as Cleric Domains, but a few are extremely good, namely Time.

[phaedrusxy]

Ardent 1 is not a bad one.
With Practiced Manifester, you can have access to 3rd Level Powers (although you would be lacking in the PP department).
Most of the mantles are not as powerful as Cleric Domains, but a few are extremely good, namely Time.

I think from the way it is worded, you actually need Ardent 2 to access powers higher than 1st level.

But all of the Tome of Battle base classes qualify for basically the same reason. Want Iron Heart Surge? Dip warblade 1 after 8 levels of other classes.

[awaken_D_M_golem]

Soulbow 1 fixes the Soulknife ... which isn't saying much.
More a matter of making the Soulknife go from useless , to viable.

Soulknife 2 / Cleric 17 / Soulbow 1 = the first Soulbow build.
Soulknife 2 / UrPriest 9 or 10 / Soulbow 1 = the second Soulbow build

(this was right before carnivore got his hands on soulbow)

[Tr011]

Ardent 1 is not a bad one.
With Practiced Manifester, you can have access to 3rd Level Powers (although you would be lacking in the PP department).
Most of the mantles are not as powerful as Cleric Domains, but a few are extremely good, namely Time.

You need two levels of Ardent. You need the first to take Practised Manifester and the second to benefit from Practised Manifester. So Ardent is not a good 1lvl dip at all.

[TC X0 Lt 0X]

Ardent 1 is not a bad one.
With Practiced Manifester, you can have access to 3rd Level Powers (although you would be lacking in the PP department).
Most of the mantles are not as powerful as Cleric Domains, but a few are extremely good, namely Time.

You need two levels of Ardent. You need the first to take Practised Manifester and the second to benefit from Practised Manifester. So Ardent is not a good 1lvl dip at all.

Was not aware of that text.
Still, it is not bad at all with some of the options it gives you with a Dip.

[Tshern]

Since we are throwing in two level dips as well, I would wish to add Prestige Paladin and Prestige Bard to the list.

[Bastian]

Martial monk is a ridiculously powerful two-level dip at low levels since you can get any fighter feat without having to meet the prerequisites.

[kitcik]

What about the fighter variant classes in Dragon 310 - it seems like they also give bonus fighter feats with no pre-reqs?

[they say their abilities replace the related fighter abilities, and the bonus feat section says nothin about meeting the pre-req's, as does the fighter's]