Tough call. I think slow time is the coolest but it is so ridiculously powerful even at level 1 that I don't like using it. I guess become ethereal it has great oh crap factors and it lets me do fun things like jump off cliffs.

Elemental Fury, and Marked For Death

Clear Skies. Helps on those annoying blizzards.

Slow time is my most used shout. (I play as a mage) I love the effect, makes it easy to dodge attacks and be precise with attacks like firebolt.

I have a lot of fun with Whirlwind Sprint though, even though I hardly use it in combat situations. It's just nifty to rush across gaps and rivers etc.

Witcher recommends Fus Ro Dah Shout
http://www.youtube.com/watch?v=JYYleOIFVyE

Dash for practical use, who seriously doesn't use it?

Frost because it looks funny, force if on a mountain or something.

But I'd say ghost mode is the most useful for skirmishes.

I get a lot of use out of Whirlwind Spfint, Ice Form and Etheral Form but Unrelenting Force is the most fun.

Unrelenting Force is handy for everyone. I love blowing enemies off cliffs and mountainsides. XD I also find myself using Aura Whisper and Kyne's Peace quite a bit.