Not my favourite way but it certainly made me laugh.

In CoD4 (or any CoD game with a small room) multiplayer, knowing there's at least 2 enemies in the next room. Standing outside, cooking a grenade, then charging in (still holding the grenade), letting the panic ensue as they trying and fire from the hip to get me because they think i'll get them first with my gun, realising that there's more like 4 enemies in there and the grenade exploding.

Would hate to see that killcam.

lol, that is funny

In Super Mario Bros. 2, World 1-1:

There is a part where you take the back entrance to get to Birdo. You have to bomb your way through a wall to get to a door.

I always toss the shyguy down the ladder shaft, and after I bomb the wall open, I drop one more bomb down the ladder, and exit just as it explodes.

Poison.
After it you don't have to clean up.
And a great addition to this is a big barrel filled with concentrated hydrochlorid acid.

poison, lol that sounds....sneaky...

Gears of War 1&2 - Chainsaw, cutting people in half = fun

Half Life 2 - Gravity Gun + Saw Blade, see above

COD:WAW - Bayonet, The look on their faces is priceless

Fallout 3 - Mine/Grenade during pickpocket, need I say more

God of War 1&2 - Shoving the Blades of Athena down a Centuars throat, sweet

V.A.T.S. Head shot :)

Stabbing in many non-vital spots with a fork until they bleed to death....

 Fixed. =D

Burning people with acid would be cool.

Until I see that in a game I'm happy to fry them with a flamethrower.