Mystic Fire Knight and Underdark Champion.

[Metahuman1]

How good are these paladin variants? If you jacked them on top of, say Paladin of Freedom and Charging smite would it be enough to raise the Tier of a Paladin?

[Water_Bear]

Mystic Fire Knight is amazing, and while I personally don't like Underdark Champion it has been widely hailed. With some other ACFs, some good rolls for ability scores, and the feats Serenity and Sword of the Arcane Order you can really up your Paladin's game.

But it won't change the Paladin's Tier, because Tier is supposed to be independent of Optimization. A well-built Paladin, like a well-built Fighter, can contribute at all but the highest levels of play, but the classes still have less potential power than a Cleric or a Warblade.

[eggs]

A well-built Paladin, like a well-built Fighter, can contribute at all but the highest levels of play, but the classes still have less potential power than a Cleric or a Warblade.
I agree until this part. Just access to shared Polymorph and Celerity provides the Paladin more potential power than high-level ToB abilities like an extra full round attack or a few extra damage on a Heal spell.

But ignoring the Tier system, Mystic Fire Knight gives the Paladin some good tools (the second and third sub-levels are no-brainers, the first depends somewhat on build specifics), and Underdark Knight at least isn't a definitively bad option (but I'm a huge fan of the mount, especially VoP griffons). Neither of those variants will optimize a Paladin on their own, but they could be useful in building Paladins that are optimized, and which can meaningfully contribute alongside caster builds.

EDIT: Does Charging Smite + Underdark Champion mean we're talking about making Paladin ACFs into free Paladin add-ons?

[Metahuman1]

Both as free add on's and as RAW AFC's.