However, in the conventional sense of weaponry:
Long Range: Barret lifght .50 (generic choice).
I can't hit a dinner plate from a yard away with a shotgun but I excel at using rifles. Once I get a feel for the weapon, I can do way better than what you would normally expect for a gun deprived canadian. Like to the point of going to a wildlife federation shooting range and being the last person done their 10 free founds of .22 rimfire ammo and having being the only one to even hit any of the ones furthest downrange. I'm not sure how far they were but from my distance, they were about half the size of my fingernail. With ironsights.
Assault: p90. Definetely. Not only is the recoil almost non existant for an SMG, the configuration genius, the armor piercing abiliy astounding and the accuracy marvellous, it shares ammo with my chosen handgun.
Self defence: Five SeveN IOM. Honestly, who needs something this awsesome for self defense? This is an assassins weapon. It's tiny and compact and weighs like a feather, yet holds 20 rounds of ammunition that can go through a kevlar vest at 300 yards. Add a threaded barrel for silencer attatchment, a scope mount and a flashlight/laserpointer mount and you have one badass gun that fits in your pocket!
Melee: Screw martial arts. Let's see them do shit about my claymore!!!
Nah, honestly, while that WOULD be amazing, I doubt I am strong enough to swing one around more than a few times. Best to go with something a little lighter. Not knives though. I hate knives. It's a real clusterfuck trying to jam it into someone. A nice middle balance seems in order. A warhammer! Nah, you can turn the head away with good leverage on the haft. Ooh ooh! A flanged mace! Yeah! Equally effective no matter where you hit them, goes straight through armor, one haded and quick to swing but heavy enough to crush someones skull and collpase their face inwards. Just thinking about that makes me shiver a little bit but I'm reasonably sure I'd kick ass with one! Plus it leave my other hand free when I need it. Perhaps for the 5-7.
Completely Silly/Destructive: Tank. Mounted. Flamethrower. Suck 1200* centigrade Napalm-B bitches! That or one of them new fangled dragonfire launchers. SWEET!