-Everything that comes out of a low intelligence Vault Dweller/Chosen One's mouth is gold. The Courier too, but sadly not
all of his dialogue is changed. If I had to chose just one conversation from all these, I'd have to go with when a moronic Vault Dweller encounters Harry the super mutant. They understand each other.
-Honestly, Harry's a riot even when speaking to a smart VD. My favorite line is when he refers to "Lou Tenant", and he goes on about how he thinks he could take him on a good day.
-Dead-Eye Ricky from Honest Hearts is another great one. He's such a minor character, and yet you get a ton of (equally hilarious) ways to deal with him.
-Getting chewed out by Sergeant Dornan.
-Throwing Oliver from Hoover Dam and listening to his over-the-top, drawn out screams. That cutscene really caught me off guard the first time I did it.
-Everything that comes out of Myron's mouth is funny. And his fate is a delicious flavor of ironic dark comedy.
-[Intelligence < 3] "Ice Cream!"
-Most of Raul and Cass's dialogue, serious or not, is sprinkled with tons of sarcasm and dry wit.
-[Sneering Imperialist] "I've heard enough. Joshua, put a cap in General Gobbledigook." It's such a mercilessly dikeish thing to say that I can't help but laugh at it.
I think that New Vegas is overall the funniest game to me. OWB aside, it is like the first Fallout in that it's serious enough overall that the funny bits usually catch you by surprise, despite them being fairly numerous. Fallout 2's funny and wacky throughout, so few things really stood out to me (those that did, like Myron, were great). And 3 seems pretty much devoid of any non teddybear six-related humor. I know there were some gags in 4, but can't for the life of me think of any right now, so I guess they can't rank among the "funniest" to me.