GOOD UNITS
Yep, the Orc Shaman is a kick ass unit. Not many units can do ranged attack AND melee, but this sucker can. Thumbs up.
In mid game I find myself unit whatever is available. I used to use Royal Snakes and Griffins a lot, but there's no supply in Kordor so I gave up on them eventually.
Pirates and especially Sea Dogs are pretty good in Western Islands if you have Mirabella.
Evil Beholders are great fun, especially their mind control trick.
Giants are okay too, especially if you've got 6-7 of them and plenty of mana for healing/defenses.
A big stack of Swordsmen or Guardsmen is nice, but after you sign the Dwarven peace treaty the main castle has a HORDE OF HORSEMEN! A stack of 70 of these is just mayhem on four legs.
CRAPPY UNITS
I didn't like the Canoneer for some reason, and Miner's really suck.
And don't get me started on Priests and Archmages. I guess you need a really high intellect for them to be worthwhile.
I never liked the Thorns or Peasants, although I hear that some people have made use of them.
In general I never go for the lowliest unit in a particular class, like the crappy snakes or crappy spiders.