This is my dakka, there are many like it, but this one is my favourite...

[FireHazard]

I'm going to have to say the Volcano Cannon. Brutally effective in Apocalypse and always a favourite in Epic. Plus the early fluff for it was great:

So, Master Techmage, what have you been up to today?"

"Not much really, but I did get bored and strip down a defence laser and stick it on a tank. Fun times."

[walrusman999]

Lascannons, because when I get enough of them together its like the drill gun from the movie The Core. And I can just punch holes in everything. After that its mostly plasma cannons and plasma guns. With a little melta in there for those few guys who need to suicide into a landraider.

[Demonus]

For pure Dakka? Vulcan Mega Bolter....while stationary. Close second is the gun the Ork Stompa fires, that keeps firing, and firing, and firing...

[Warptiger]

Autocannons are fun.

I used to use giant packs of chaos cultists with 2 autocannons. Blew up a leman russ once. If I remember right, back then chaos cultists needed a 6 to even hit. It hit, it penetrated, he rolled badly, and the tank blew. We both sat there in stunned silence. I felt so bad for my opponent, I apologized.

I mean, they were chaos cultists. They're weren't supposed to actually hit anything.

[Zac78]

+1 d cannons

[Dalleron]

I have to say that I have become fond of the pulse rifle. Strongest basic infantry gun in the game. Had a full squad shoot up the rear end of an ironclad dread once. It was good for me, not so good for him. And today, they are so much more fun.

[Mr Mystery]

I do miss flamers in 2nd Edition though.

Nothing like popping up near some Guardsmen, giving them a quick dousing, and watching the various survivors run around pell-mell burning merrily away Happy days, but very daft rules!

[L192837465]

Shokk Attakk Gun

My god, everything about that gun is pure genious.

"I'mma teleport a grot into your fusion reactor! Hah!"

[Psychosplodge]

Wouldn't that just vaporise the grot? Now try to get snotlings to occupy the same suit of power armour as the spacemarine wearing it...

[Learn2Eel]

That's a tough one. If we are counting ranged weapons as just guns, probably the Baleflamer - mostly because I love Dragons
Otherwise, it would probably be Psychic Shriek - the image of a mind being ripped apart by a scream no one else can hear is delightfully disturbing.