Adjusting to the situation is important. Many good ideas have been mentioned already; here’s some suggestions on when to use them:

Fall! or Trip!

Golden in precarious terrain or situations. I once had particular success with “Fall” against a charging guard captain… as he was mounted on a warhorse at the time. Leading the charge.

Also, instant death threat against anyone who’s climbing something…

Undress!

As Jeff pointed out, golden against anyone in heavy armour. Will keep them occupied most of the duration, and they’ll be easier to hit afterwards.

Surrender!

Seeing as most people won’t feel bound by a mind-controlled oath, “Surrender!” is useful primarily if you’ll have opportunity to take their weapons or tie them up before it wears off. (Note: If you use it against honourable people who were already feeling intimidated, they might never realise you used magic! Against a commander of knights or the like, you can take out a whole unit this way…)

Sleep! (or, equivalently, Die!)

Buys you one free round. But there are better alternatives.

Vomit!

As disabling as “Sleep”, and lasts the full duration. Messy, though.

Flee!

Effectively doubles the duration – if they spend the entire time running away, it’ll take them just as long to come back. Best used against key members of attacking groups. One key character leaving lets you deal with the rest while they’re gone.

(Panic! can be just as disabling, but it’s very unpredictable. Some people are actually quite dangerous to all present when they’re panicking…)

Talk!, Lecture!, Sing!, Silence!, or for bonus points, Monologue!

As they might fail the save, golden against low-level mages. (High-level ones are too likely to save; don’t risk it.) A skilled villain can talk and fight at the same time… but a mage really can’t.

Dance!

Disrupts activity better than “Sleep!”, tiring, and disrupts mages nearly as well as the above (Somatic components!), plus very funny. (For bonus laughs, name a specific dance instead. Waltzes are good. Two clerics together can really mess up your opponent’s morale with a well-aimed “Tango!”.)

Help! or Cooperate!

Very risky here; use only if it’s utterly unambiguous what they should help with. Never use with more than one enemy present… they can just choose to help each other.

Swim!

Great in the presence of pools or rivers of any liquid. Takes them out of the fight and leaves them in a bad position when it wears off. And against opponents in metal armour, implies “undress” for free, since they won’t commit suicide.

And beware the GM looking for excuses to disallow commands: The book is explicit that “Suicide!” doesn’t work because it’s a noun. Of course, “sleep” is a noun, so the rulebook contradicts itself a bit here. Depending on how mean your GM is, so is almost everything else on this list. (This is why “undress” is better than “strip”.)