For me this isn't even close.
It has to be Pie Girl Kay's 'Hit me with a pie!' in her Fan Mail(or was it Flan mail?) vid. This never fails to turn me on!

I have some favourite lines, but they aren't necessarily hot when you take them out of context/aren't me:

"No, no! Not me!" Pie Wedgies

"Ever had a Lime Sandwich?"
"No, what's that like?"

"But I'm littler than you!" Heather vs Paris

"Hey Liz, it's time to take your shirt off!" Lexi, Messygirl

"Ahh, the old pie-in-the-tits!" Kymberley Jane

Actually there are loads that I like. I might try and recall some more later!

Least favourite:

(delivered from off-camera in a loud whisper)

"Look up. Look UP"

"Say 'Let me have it'"

How about the traditional debut line on MG's site, "I wanna be a Messy Girl"?

"Let me have it." - MG's Sara

"Do your worst." - Phoebe

I'd have to go back to it to get the exact line, but there's a Penelope/Tilly video where they make out/get sexed up for a while on a couch (super hot), and then they kiss and Penelope says "Let's go check out those pies..." in a sultry, intimate way that blew my mind.

It's one of the closest "porn-as-wam/wam-as-porn" videos -- and lines -- i've ever heard.

Also, i always found it super cute-hot when Phoebe R. would say, "You wouldn't/wouldn't dare" in a matter-of-fact way. And then got splatted.

JY

I think the line is less important than the girl saying it... and the hit to follow.

We got a LOT of mileage out of, "Stay tuned to watch 4 girls get creamed with a PIECE of PIE." SPLAT! "Make that FIVE girls...."

The line from this early Messygirl sample is etched pretty deep into my brain after countless repeat plays

Liz in Messygirl's pie target series, "Go ahead, pie me!"
Liz in the same video, "Give me more!"
She had such a sexy voice and sounded like she really wanted it.
Messygirl also had a one and done blonde Michelle. Her line "Got anymore of those lies for me?" Followed by her "mmmmm" really does it for me.

That Rob Blaine video ("Mud Bondage") where Celeste is whining about being tied up in the mud and then Hara lifts her face out of the muck and says, mud dripping out of her mouth, "It could be worse."

Mainstream scenes probably don't count but you gotta love "Now I'm in my own natural element" and "And don't forget you said that!" (from THE BIG DOLL HOUSE), and of course "Into the mud, scum queen!" (from THE MAN WITH TWO BRAINS).

Also on a permanent loop in my brain, even though it's not attached to a wam scene: That PERSON OF INTEREST episode where they have to hide in a sewer and our girl Sarah Shahi says, "All right, boys -- into the muck." As if she's talking directly to us!

I cannot name specific video titles, but I know I've heard in at least 2 pie-videos, after the girl gets totally covered in the face with pie, the girl say:

"This is totally humiliating."

I got dressed up just for this i just hope i don't get my white pants dirty