Unrelenting Force for me, I love that shout.

Hands down become ethereal, lets you cliff dive from ridiculous heights, its also basically a panic button during combat if for example a dragon starts breathing fire just become ethereal and laugh at them. Also great for getting through spike/flame/swinging axe traps. Relatively short recharge time too.

I reckon more people would use it if it was more correctly named 'become invincible'

Without a doubt Storm Call. I love finding a big forsworn camp in the mountains somewhere sneaking in with invisibility, casting Hysteria, then conjuring two Dremora Lords, then Call in Storm Call, all the while calmly walking through the tents frying everyone hiding with dual lightning bolts. Become Ethereal is awesome for master destruction spells. I'll wait for a group of Draugr to circle around me and come in close, use Become Ethereal and be impervious to damage and staggering, then charge up fire storm (i think thats what its called, the one that shoots out a circle of flame), and watch the chaos. Slow time is good for fireballs and frost dragons too.

Unrelenting force is the only one i use so far. Have like 8 dargon souls and a few words. Havent found one better or funnier the UF.

My favourite is storm call but you can only use it so much.

My default is unrelenting force (usually just fus to stagger ennemies enough), but I also use slow time, become ethereal and whirlwind sprint. I use other shouts as well but less often: disarm, marked for death, etc.

I've learned to appreciate shouts a lot the more I play. There's one for any occasion!

Become Ethereal because I love jumping from high spots and it also allows me to sprint longer.
Clear Skies because I like to hunt flying insects and they dont come out in bad weather. It also allows me to see the most beautiful vistas that otherwise would be obscured by the weather.

Become ethereal, it lets me take shortcuts from high places

Absolutely love Slow Time. Pop all three words, smash 5 or so mobs before they land a hit, casually sheathe sword just as effect is ending. Epicness

Unrelenting Force is the most versatile, since it effects just about everything.

I also use Fire Breath, seems to do a good amount of damage. (Lessens in higher levels sadly, it really should level with you...)
I use Aura Whisper a lot on my thieves, since they don't do casting much and thus don't have the "silent casting" perk. People say Illusion is vital but I've had multiple 40+ thieves without needing it.

Unrelenting Force near an edge always gets a smile out of me. During combat I use marked for death, become etherial and stop time the most.

Whisper I miss detect life

Fire Breath. The words of power for that shout sound cool lol.

OH MY GOD A HAGGAR AVATAR. NOSTALGIA TO THE MAX.

Kyne's Peace, I found it very useful when I came up against a Spriggan and she called all the local wildlife to her aid. Just as they were about to attack and do her job for her I sent them all packing and evened the odds.

Storm Call, Unrelenting Force, and Animal Allegiance, in that order.

Detect Life Spell is still in Skyrim. It's an Alteration Spell.

Ice Form and Unrelenting Force are the most useful to me, but Storm Call is the most epic.

Using Ice Form against multiple strong foes is just awesome, while Unrelenting Force shines against single enemies.

STORM by faaar, camp on a high rock use the shout and just sit your ass until everyone is dead or weak enough to be killed in one hit

Stormcall, then Slow Time.

Storm Call is awesome but I hate the recharge time.

I think my favourite would be another vote for Storm Call, especially with the Immersive Thunder mod - that really feels like powerful magic being wielded. But it's so damn powerful I've only used it once in battle, when I'd left Lydia and horsie well out of range. Most of the time it's just not practical for me. The Shout I really rely on and use most often though is Aura Whisper, as I like to know what's around the corner. It can be tricky to use, though; the other night I was edging down an unfamiliar hillside in The Reach, hit AW to see what was over the next rise, and saw a load of lifesigns off in the distance. Was it a Forsworn camp? A large pack of wolves? I crouched, sneaked to the edge of the hill, and saw...a farmyard full of chickens about 10 yards away.

Brilliant!!!! Thanks for the responses people have given so far. It's great to know what shouts the citizens of Skyrim use.