Here's mine. (Not MY gamertag, but the funniest I've seen.)
"me nob"

RHCPfan24

BillCosbysSack

I haven't found a funny one yet.

Howd_mydiktaste

" Here's mine. (Not MY gamertag, but the funniest I've seen.)"me nob" "

There have been a couple that have made me laugh. Probably one of my favorite though is "A Sandy Mangina"

The two that really got me and I still remember: drop ur panties and obamasukscok.

Found this from Kotaku. Pretty funny names on there.  Funniest I've ever seen personally...can't think of one. :\

LILkitten_stomper (I'm not sure if there was an underscore, and of the caps, but you get the jist of it)

BashHerFace

My friend from giantbomb "Your Fat Uncle" has a great name.

I once saw someone named "My Wife Is Fat" (or something like that) and in the motto it said "she is ugly too". I laughed pretty hard at that.

SmokesMADBlunts69

Just because its a perfect storm of stupid.

I was playing COD, and I got sniped down. It said: 'Killed by YourMumInAmerica' I laughed pretty hard at that.

HornyFetus, it was a pretty fucked up name but I laughed when I saw it. It was also hilarious saying things like "nice kill HornyFetus!"

Manfistingfiesta or something along those lines made me laugh

se_ygrandma43
you know the drill

If I was on Live, it would be the same as on PSN.....mary_effn_poppns

ILURK4CHANLULZ

For the majority of you who get that, I just thought it was really stupid. So I laughed.

WombBanger was my favourite. Though he had to change his name eventually lol

"C4 in your pants" and "your drunk dad" are the only ones that stick out to me.

You were killed by...C4 in your pants

assfacemcblojob

I saw some dude just called "Your Mum" a while back, not sure if he got banned or not though, found it pretty funny.

KKK42069

I played a L4D match against "TinyYoda, CAPSLOCK_LOL, and Mr. Mooncricket. Although the last is incredibly racist I still got a laugh out of all of them. 

Wilma Fistfit

dungeon keeper avatar :D i haven't played that game in ages

xKNOBHEADx is a classic.

dirtyassasian

at first we were saying assasin but its dirty ass asian

DudeFluid

I saw one that blew my mind. The things people do these days to be "cool"

I keep a running list on my phone, here are the highlights:

mofugga
memberthat1dude
Ultimate Gandhi
Seismic Wanker
BigGulpMADNESS

It's not a gamertag, but for the longest time, my WoW arena team name has been "Come honor face"

My kids thank you all for making Live a place they can't visit.

" I pwn vachinas"

I guess they aren't funny if you don't get the references, but I saw one that was "Dr. RoxXxo" and my current one makes me smile every time I see it " Pepe Silvia"

type the later into youtube for a little laugh I-)

" @RipTheVeins:

Im Doctor Roxxxo!!! I DO COCAAAIIINE!!!! Seriously, lotsa cocaine.

"

"Tyrannical Tbags"

Today'sEconomie,something like that.

Just imagine,you've been killed by Today'sEconomie!

" My friend from giantbomb "Your Fat Uncle" has a great name.I once saw someone named "My Wife Is Fat" (or something like that) and in the motto it said "she is ugly too". I laughed pretty hard at that. "

i just saw one on PSN called, "I_AM_WHO_I_AM"

my friend "mumtoucher"
 
I remember trying to talk to him on games, you sound like such a fool going i need backup mumtoucher!

I used to have for gamertag "TwoHairyBall"