The following are in no particular order.

#1: Ronald McDonald- He owes me a free burger, but won't admit it.  When I find him, I am going to beat it out of him.
#2: Street corner evangelicals- If I am going to hell, I might as well enjoy myself on the way—by punching you in the genitals.
#3: Bears (black)- Scary
#4: Bears (polar)-Scary
#5: Bears (brown)-Scary
#6: Bears (Panda)-Scary
#7: Bears (Koala)-Scary
#8: Bears (care)- Bad personal Experience
#9: People who quote borat- Its not that I don't get the whole joke behind Borat, its just that I hate when you talk like that.
#10: It looks badass.
#11: The ghost of richard nixon- Legend has it that he roams the streets at night looking for unsuspecting pedestrians to engage in fisticuffs.  I don't want to be caught unprepared.
#12: Republicans- Politics make me angry
#13: Democrats- Really, really angry
#14: Car salesmen- smug fucks
#15: Other people that wear brass knuckles- gotta keep that playing field level
#16: Pretentious emo kids- improves my odds of breaking their glasses by over 50%.
#17: Ninjas- Though I would certainly lose this fight, I want my last moments to be interesting at least.
#18: The Pittsburgh Parking Authority- They descend from rooftops on cables and give me tickets even though I only ran in for, like, five minutes and I totally had my flashers on.  seriously, what the fuck?
#19: Cheaper than silver knuckles- Though these are favored by many of europe's finest hooligans, I simply cannot afford this luxury.
#20: The guy who drives the bus I ride to work- I think he would look better with no teeth at all, as opposed to the two rotten, yellowed stumps he has now.
#21: The better to backhand my girlfriend with.
#22: It's part of my heritage- my great grandfather was a maker of fine brass knuckles.
#23: Clowns- Scary
#24: PETA-  Actually these aren't brass, they are made from whale bones.  You want a bite of my burger?
#25: Simon Cowell- baby gap t-shirt wearing douche.
#26: The mob- I may or may not owe someone 20 large (some body please hide me).
#27: Children- there is a discipline problem in this country and I aim to fix it.
#28: It makes my threats more meaningful.
#29: Clay Aiken- Like he wouldn't be on your list.
#30: Robots- If the Terminator movies only got it wrong by a decade, SkyNet might already be online.  I'm not one to take chances.
#31: Nuns- I don't want to punch any nuns, I just need to protect my delicate fingers from their merciless assault.
#32: Bears (all other, including Chicago)- Assorted Varieties of Scary
#33: Global Warming-  I am going to find a way to punch the atmosphere so hard it restores the ozone layer.  This is my idea so don't steal it, you assholes.
#34: The Kids- It turns out being a smartass is as or more dangerous than smoking.
#35: Hippies- You don't believe in violence? oh man, this is going to be easy.
#36: That creepy guy who rides the elevator all day but never gets out and smells kinda like rancid meat and stands too close to you and makes you feel really uncomfortable- Scary
#37: Takes kinky sex to the next level.
#38: Every once and a while you get that "clang" noise when you punch someone, like in the movies.  Incredibly rare, but worth it.
#39: The Family Circus- Whenever I read this comic, I want to run towards the nearest living thing and kill it.
#40: I put them on yesterday, and now they are stuck.