As Draco said, it really depends what you're doing with them. I use different units in different circumstances, but here are the ones that I like, and why...
Frogs and penguins are good for taking and guarding things on islands and in the water. But rarely useful otherwise (except that I love that noise that penguins make when they get hit, and it cracks me up to build stacksfull of penguins just for the look of it).
Boars are only mediocre backup/scout unit to fill out a stack (use them to join your hero or leader when far from reinforcements), but I would only bother using them when I don't have the mana or know the spell to summon something better. Pony riders and hell hounds both make excellent low-level scout/cavalry.
Elven/dark elven archers are pretty good marksmen... I'd even take the fair one over a ranger most days, but not over a centaur. I also really like halfling slingers and goblin darters.
Orcs are good swordsmen, and dwarven beserkers are fun, but their defense is so low they are a little bit disappointing. Kobolds are nice, especially in numbers, with their poison strike, but goblin darters are better. Spirit puppets are great to have around for wraiths, etc. I quite often use human pikemen as an extra fighter-type in an army.
Nymphs and ladies of pain are great for convincing wary heroes and other desirable units to join your cause.
I am way into elephants... I think they are probably the best level 1 unit in the game. I'd say I tend to favor elephants, and any ranged unit. My favorites are the the elven/dark elven archers, halfling slingers, and goblin darters.