Aetius,

The best would be to discover for yourself, as it opens your eyes to different possibilities with all the other races. But in the spirit of help and friendliness there are a few points I'd hope to help you with.

A good number of the "Human" races are among the most powerful. Man, Pithium and Arco are all among the top all around contenders, though they don't all play the same.

Most of what I'll be saying are some tips to utilize in your own personal playstyle and game situation.

Man:

Knights and KoA. Pricy, strong troops that move fast and can be used to break an enemy or hold back a line or fight those summons along with their Longbow backup.

Longbows. Cheap, effective, good precision, fast to build. Longbows also make very effective Indy killers and sacrificial troops. Also forces opponents to use counters (Storm spell effects, Arrow Fend, etc) for them, otherwise they will be at a disadvantage. Able to pierce heavy armor fairly well especially with the number of troops you can produce.

Crones. A cheap way to make a disposable army. Crones have access to Air and Nature magic. Nature 3 in Fact, which will allow them to summon Vine Ogres (3 Nature, 1 Nature Gem). Equip 4 or 5 crones (Or more!) with Ivy Crowns, and later, Treelord Staff's and set them to monthly ritual of Vine Ogres and you have yourself the heavy disposable troops to use in tandem with your longbowmen. You can field incredible, mobile armies with Vine Ogres with minimal gem usage. Very valuable. Then for combat they have access to Air and Nature making them highly precise (Eagle Eyes) have good spells like River of Life, Orb Lightning, Regeneration, Haste, etc. For a variety of uses to cripple and kill your opponent while buffing your own troops.

Wardens, Monks, Bards, Mothers of Avalon. Ever wanted a stealth army? Covert ops and behind the line fighting more your style? Welcome to Man, only matched by Pangaea for the availiability of good stealth units and commanders. Proper use of these types of troops are invaluable and can turn the tide of a war easily.

Last but not least, Bards (again). Bards are highly stealthy, can instill uprising, have valuable songs, cheap (75 Gold), research, 1 Nature, can move 2 and are buildable outside of the capital. These guys are great for support and intel.

That is just Man, and that is only in one playstyle, there are a variety that work and one of the best things, is the ability to adapt.