Since chum got to show off his army, I thought I'd do mine.
If my name wasn't a dead giveaway I play Tyranids, they are my favorite army in the Warhammer 40,000 universe. However the status of the Tyranid army is lack-luster at best, and if the rumors about the spacewolf codex are true then their may be no reason to play Tyranids at all competitavely. All that aside, I use a Godzilla list when I'm playing competitively. You know the typical 2 walking tyrants, 3 elite fexes, 3 Zoanthropes, 2 big fexes, and some genestealers.
Most people use ripper swarms to take up the required slots that a carnifex cannot fill, but I go for genestealers. Genestealers are my favoire Tyranid unit (once again as my name would imply), but thats not the only reason I use them. What I like to do is hide a small unit of genestealers behind my carnifexs, blocking them from line of sight. So if any nasty powerfists wanna get to close to my Carnifexs they'll be dealt with; genestealer style.
The walking hive Tyrant is a staple of any competitive Tyranid army, it has the potential for best BS, and lots of good weapons to go with it, my favorites are probably the devours, cause the venom cannon doesn't have enough shots to properly kill stuff. Also you really can't go wrong with tyrant Guard, give them some lashwhips and the tyrant is covered from shooting and combat.
Ahh the Carnifex the crowned Jewel of the Tyranid army list, it's big, bad and relatively cheap. For an elite fex I usually use the barbed strangler, it's a big ordnance weapon, and thanks to fifth it'll always go off, it may scatter but that's better than having both your venom cannon shots miss. For my big Scary carnifexes I always use extened carapace, which I rarely ever see in other Tyranid army lists; IT'S 15 POINTS FOR AN EXTRA WOUND!!! come on guys. I really don't bother with Carapace their aren't many AP3 guns anyway. Other than that Senses and Barbed strangler & Venom Cannon combo, just cause it might hurt tanks, though probably not.
Zoanthropes, are not necessary to my army, but they do have a strength 10 blast that isn't crap like the venom cannon. So I like to keep them on the off chance that they'll blow up a landraider. Also for 55 points each it really isn't that big a deal, besides you always have the small blast to clean up and heavy troops.
So through thick and thin I've played Tyranids, and I hope to in the future. But that doesn't mean I can't play other armies as well.