Level 1: Stratos
Without a doubt, cos Frostwolves are fast, aswell as their speed up ability, and in packs they can be really annoying. Plus lighting only costs 200 mana, and it recharges quickly.
Level 2: Persephone
Scarabs are good, cos you can't always be healing your creatures in the midst of a battle, so they're extremely useful for this. Also, ethereal form COMPLETELY makes you invincible.
Level 3: Persephone
Once again, because this time, it has Trolls which are great at destroying manoliths, not to mention the evil grasping vines which are convenient to use on your opponents and then kill 'em so you can convert their souls easily
Level 4: Persephone
Yet again, Persephone has to be first choice. Why? Simple! Rainbow = PWNAGE. Gnomes are deadly against flyers, but when guardianed, can be a threat towards ground attackers, too...
Level 5: Charnel
Demonic rift is easily one of the best spells in the game because of it's easy damage, and deadeyes are...well...deadly. lolz. Anyways, if you make tonnes of them and line 'em up or cast guardian on them, the best *PROVEN* strategy to defeat them is mass Frosties.(frostwolves, for you noobs>:P) So they can't be stopped that easily....
Level 6: I can't honestly remember..lol...sorry/soz folks :/
Level 7: Charnel
STYX and stones...wait, forget stones, styx's will f***** kick 'yo ass, biches! One of the strongest creatures in the game, having a handful of these COULD mean certain victory. Unless it all goes wrong. also, plague is a great diversion, or a way to weaken your opponent's army.
(Pyro is also a good choice here, because Warmongers and the spell "Rain of Fire" are very powerful)
Level 8: Stratos/Pyro
A tornado + Cloudkill combo is sure to stir things up here And volcano is useful and convenient in so many ways (Not to mention the Dastardly-evil Blind Rage spell)
Soz for the spam..*ahem* now then..onto business...
Level 9: James/Stratos
Once again, a tossup. Becaauuuussseee....SILVERBACK AND RHINOK R BOTH PWNAGE P[L0;X
Sorry, i'm hyperactive, i just got bored : :