I've been thinking about this. I've heard that the Centaur Captain and the Halbardier compete for the slot of best tier 1 creature, and that Naga Queens and Dread Knights are the best of the sixth tier, but what creature from what town fills the other slots? Either statistically or in regards to whatever special abilities they have.

I've never been quite clear about the point of this question, because it looks at creature stats in a vacuum, and the creatures generally don't act by themselves. Creatures usually have roles to play in their town's line-up, which can drastically change the math on their effectiveness.

Examples:
-Familiars are really bad combatants, but they grow in huge numbers, they can help kill the enemy's mana, and throw in some Pit Lords and you have the ability to turn Familiars into Demons for that battle.

-Dendroid Soldiers are awful, awful, awful because they are so slow. Dwarves are in a similar boat (though Battle Dwarves do quite a bit better in this regard). They just can't engage the enemy very effectively. However, their town includes double shot Grand Elves, which are wickedly strong shooters unless engaged. The presence of the elves means the dwarves and dendroids have enemies coming to them.

-Master Gremlins are pretty lack luster, being a rather weak shooter. But they contribute quite a bit in a line-up that includes Arch Mages and Titans for a total of 3 shooters, with Iron Golems and Naga Queens intercepting the enemies hoping to block the shooters.

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All that said, there are some stand out creatures in each tier, if you're looking for 1-off additions to your army.
-Sprites are super fast flyers with No Retaliation. They're probably the only T1 creature really worth a 1-off inclusion, assuming you're pulling from outside your main town type. Sure, Halberdiers or Centaur Captains may have more raw stats, but they're simple melee fighters, and Halberdiers may actually slow your army down.

-Marksmen (double shot) or Harpy Hags (fast, fly, No Retaliate) are the only T2s I'd really consider.

-In T3, Cerberi are quick with No Retaliate and a multi-hex attack, but they also tend to die quickly when the enemy gets their turn. Grand Elves are good shooters, but might be too flimsy for use if you can't get them in good numbers.

-The Arch Mage is an excellent T4 creature with a solid ranged attack and the spell cost boost for the hero. Greater Basilisks can be problematic, being rather tanky and having an innate chance to disable a stack with every hit. In large numbers, Vampire Lords are very, very hard to eliminate if they are fighting living creatures.

-Pit Lords can supplement their army when allied units fall, but it does require losses in your army to use. Power Liches are a very powerful ranged creature IF AND ONLY IF your army is immune to the death cloud. Mighty Gorgons can be terrifying if they have large numbers; small numbers aren't as bad, but the unit is still tanky. Minotaur Kings get an honorable mention for being basic melee walkers with good stats.

-You really can't go wrong in T6. Wyvern Monarchs and Scorpicores are the bottom of the pack, being less tanky than the rest and having more questionable specials.

-Archangels, Titans, Black Dragons, and Arch Devils are the top of the line at T7. Phoenixes are also up there when you consider their speed and that they get double the growth in their dwelling. Chaos Hydras are probably the bottom of the pack; they are good fighters, but they are missing the speed, tanky-ness, or quality special abilities that other creatures in this tier have.

Call it curiousity. I'm not very good at the theory crafting thing, so I want to know what other people would suggest or might think about a given thing. Thank you. It helps. :)

T1: Centaur Captain has best stats, but Master Gremlins are quite good as well. Massive stacks of ranged units are undeniably useful.

T2: Storm Elemental does comparable damage to Wolf Raider, and its stats make it harder to kill, even if you exploit its weakness to lightning. Can't be easily disabled with Blind, Berserk, or Forgetfulness either. Honorable mention to the Harpy Hag being invaluable in siege defense.

T3: Tough call between Grand Elf and Dragon Fly. The elves put out serious damage with their double shots, but the speed, Dispel, and Weakness of the Dragon Fly makes them a huge tactical asset.

T4: Vampire Lords with their life drain are phenomenal, but if your opponents aren't "living" you're better off with Archmages.

T5: Mighty Gorgon's death stare is extremely dangerous with a big enough stack, but the speed and power of Thunderbirds, especially for how early they can be produced, is excellent.

T6: Magic Elemental is like a small hydra, plus magic immunity and decent speed.

T7: Range penalties and lower speed mean that Titans are no longer the top contender with Black Dragons. Archangels are of course amazing, but the line attack of dragons might still put them just above. Gold Dragons have better stats except for hit points, but their susceptibility to Implosion is a big detriment.

I like Dragon Flies and they are annoying to fight, but their combination of speed and relative fragility means they tend to die quickly. I typically find them annoying, but not exceptionally threatening. I agree about Thunderbirds. Both Rocs and Thunderbirds are very solid units on their tier, having respectable speed, damage, and durability. The nice thing about the Titans is a combination of Ranged attack and No Melee Penalty. They are probably less threatening compared to Black Dragons, Arch Angels, and Arch Devils in a direct comparison, but they complement their town line up very well.

I agree about Gold Dragons as well. I didn't list Gold Dragons in my top-of-class list precisely because of their vulnerability to Implosion.

It's not that the dragon flies are super threatening themselves, but their speed may let you cast a crucial spell before the enemy, or they could weaken a unit that would otherwise be problematic. They're also great at pinning down ranged units. The versatility ranks them highly. Dragon Flies have only slightly less HP than Griffins and Cerberus, but with more defense. Really only the Ice Elemental and Iron Golem are significantly tougher in tier 3.

Of course Titans are still great and no Tower would be complete without them, but they're maybe top five in Heroes 3, while they were a strong contender for number one in Heroes 2.