#5 - Athasian Sloth (4th Edition, 230/500)
When the Athasian sloth is played, your champion gets a huge +8 level-up bonus, plus your opponent is forced to choose and discard two cards from his hand. Unlike our #4 pick below, it is not necessary for you to lose the round of combat to use the Sloth's power. This means you might also get a spoils, depending on how the rest of the combat goes. Card advantage for you, thinning out your opponent's hand, and a whopping +8 bonus for the duration of the combat round? Nice ally!
#4 - The Starving Artist (3rd Edition chase, 439/440)
This card is very popular among Standard Spellfire players, but not so hot for TAV (well, by "not so hot" I mean it's only the fourth best ally of all time). Card discard is just not such a big deal in a faster format with tons of instant-kill cheese. For the same reason Cold Cup of Calamity is overrated, the Starving Artist is overrated. That having been said, this is still an awesome ally. You do have to lose the battle for its power to go off, but if you get lucky it can cripple an opponent. That unlucky opponent (you get to pick if it's a multi-player game) has to get rid of all but two cards, and no-one gets a spoils for that combat round. The reason it's rated ahead of the sloth is that it can potentially discard many more of your opponent's cards.
#3 - The Dreaded Ghost (4th Edition, 246/500)
#1 - Thought-Eater (Artifacts, 99/100)
That's right, folks. The only ally so powerful, they depowered it for the 4th Edition printing! He lost his ability to affect magical items, but even the 4th Edition version would be on this list. The original Artifacts Thought Eater shown above is a beast in every sense of the word. While he does have a slight downside (reducing your champion's level by 2 when he is played), even that's not too bad when playing the Antigonish variant, which is full of instant-kill cards. You actually want a lower level, so you can get off more cards before your opponent gets to play. The powers of the Thought Eater are ridiculous. Every magic item, ally, or spell played by the opposing champion has no special power (level only). The opposing champion loses the ability to cast spells altogther. And the opposing champion's special power (including all immunities) no longer functions. Turn out the lights, this battle is OVER.
Honorable Mention - Intellect Devourer (4th Edition, 213/500)
This guy didn't quite make the list, but he can be an awesome ally at times. The Intellect Devourer automatically destroys opposing champions of level 5 or less, which makes him ideal for removing Erellikas, Julios, Living Scrolls, Crawling Claws, and other annoying, low-base-level champions that seem to proliferate in Spellfire:TAV games. Slap him down, take your spoils, move on...nice and neat!
Next time: Fences!