We need to find more of these secrets 
There is no possible way I could pick favorite dialogue in a game so overflowing with voice work of this quality.
I guess the little aside in Renegade where Phoebe makes a pass at Caldarius just before they rescue him, and admits in a totally fangirlish voice that giant flying death mechas are, like, her favorite thing in the UNIVERSE.
Oh, and how if you play as ISIC in The Algorithm, he quietly gloats to himself about how satisfying it is to kill his own creation (when fighting Geoff/Arachnis).
Everything Boldur says, any dialog tree, in The Experiment. (She is dead, WAAAHAHAHA! Sorry, laughter is how Boldur grieves!)
Caldarius calling Hylis handsome.
Toby being… Toby. (Seriously, anything he says.)
Too many other priceless lines to list.
Pure gold
When Mellka kills Deande, " Nobody is going to miss you , even Montana and that guy loves everybody!"
Deande on Renegade mission about Dibs, always cracks me up.
We were here first, you DIB! Hahaha!
That dialogue is so good.
Every dialogue is pure gold in this game. But here are my favorites (not all of them. I have 180 hours of gameplay right now, and i didnt tried half of characters yet.
) (PVP quotes)
Oscar Mike:
- All units be advised, airstrikes are badass.
- I have a thousand bullets with your name on em’! I hope your name is Chuck!"
- Come on, rage quit, you know you wanna’.
- Sitrep, i was dead, it was cool!
- So i am a new clone, or umm… /when he resurrecting in PVP modes/
Montana:
- Oh crap… guys? Im out of ice jokes.
Toby:
- Hey other Team! I left something here for you! Its a CAKE! I SWEAR. I SWEAAAAR!
- I cant wait to have nightmares about this later.
- Oh man, sorry about your guts!
Whiskey Foxtrot:
- Still alive? Good, because im still mad!
I healed Yoby the other day from near death as Alani, and if I recall he said something along the lines of “Oh hey, I’m still alive. THANK YOU” or something similar. It got a chuckle out of me.
#Alani
- “F%$# your sandcastle!” - Using Riptide
#Attikus
- “U W0T” - Being selected in Character Select
#Benedict
- "Print “tried my best” on that guy’s tombstone. With crappy font. AND GLITTER AND S#$%!" - Killing an enemy
- “Get’chur self acquainted with rockets!” - Activating Hawk Eye
#Caldarius
- “I HAVE THAT SAME DRESS!” - When spotting himself/Phoebe(?)
- “Oh, yeah. He’s pretty handsome.” - Response to Jailer Hylis on the bridge part
- “We did the same thing last year, but Data Recorder doesn’t seem to mind. That’s what love is, you know.” - Caldarius’ Lore Audio
- “FLIGHT PATTERN: BADASS!” - Jumping from a Jump Pad
#El Dragon
- “Damn these majestic, muscular, ROBOT ARMS!” - Said during “Training Regiment” Taunt
- “Pinned by submission!” - Killing an enemy
#Ghalt
- [Psuedo Voice]“Holy s#$%, Ghalt’s got two guns![Regular Voice]Yes’sir, I’m afraid HE DOES!” - Activating Double Wield
- “Victory will be “mine?” Eheheh…Yeah, screw you guys, too…” - Planting a Scraptrap
- “♬ Step into my office-ah! ♬” - Activating The Hook
- “♬ Here comes the HOOK! ♬” - Activating The Hook
#ISIC
- “Just try to kill me! Hahaha, I f#$%ing dare you!” - Activating Rotating Wards
- “Get f%$#ed, underling!” - Killing a Minion
- “I prefer human shields, but these will have to do!” - Activating Rotating Wards
#Mellka
- “Suck it, Caldariuuuuuuuus!” - Jumping from a Jump Pad
- “Benedict suuuuuuuuucks!” - jumping from a Jump Pad
#Orendi
- “MUH CORNEAS!” - Being affected by Blind
- “I had to use ALL my arms for that one!” - Killing a Major Enemy
- “[Sobs] This suuuuucks, help meeeee!” - Asking for help when Downed
- “Nooooooooooo ok.” - Getting Downed/Killed
#Oscar Mike
- “Today’s weather is cloudy with a chance of SICK-ASS SPACE LASERS!” - Activating Space Laser
- "Yup, that was me. ALL me. THIS GUY!!!" - Killing an enemy
- “Okay, I know this looks bad, but it’s actually REALLY REALLY BAD, PLEASE COME HELP ME, AAAH!” - Asking for help when Downed
#Pendles
- “That jaw bone would make a great door knocker! Someone grab it!” - Killing an enemy
- “And just like that!..I’m gone!” - Using Smoke Bomb
- “You’re one easy mark, mate!” - Said during “Slither Slayer” Taunt
#Shayne & Aurox
- “Yeah, nice one, Champ!” “STOP CALLING ME THAT!” “Uh…Slugger?” - Killing an Enemy
#Toby
- “Lets ride, Death Machine!” - Killing an enemy
- “Back at base! Safe & sound! ME & DEATH MACHINE!” - Respawning in PvP
- “Oh, he’s sooo cute-argh, dammit!” - Spotting another Toby
- “I warned you! Did I warn you? Consider this a warning! I’M SORRY!” - Killing an enemy
- “You okay? Hey, IS YOUR FRIEND OKAY!?” - Killing an enemy
- “I’m a frikkin’ surgeon with this Railgun!” - Killing an enemy Battleborn with a Critical Hit
- “[Sobbing]I’m giving them so much XP!” - After respawning from death in PvP
- “You see any brakes on this walking death machine!? NO BRAKES! NONE!” - Getting a kill streak
#Whiskey Foxtrot
- “Gonna…kill you…SO HARD!” - Being affected by CC effects
- “Target pacified. That means KILLED!” - Killing an enemy
- “If this doesn’t kill you!..Uh…Tetanus probably will!” - Activating Scrap Cannon
- “LET’S F#$%ING DO THIS!” - Activating Overdrive
- “TICK! TICK! …Probably another tick…BOOM!” - Activating Sticky Grenade
- “Ever seen a clone with an ass like this? Mmmmm!” - Said during “The Tea Ceremony” Taunt
- "Haul ass! Roast noobs! GET PAID! BUY STUFF!" - Jumping on a Jump Pad
- "Majestic! As! F#$%!" - Jumping on a Jump Pad
- “Who’s defective now!..Dammit, it’s still me!” - Gaining a Level
- "Duty called. Left a message. It says “You got this.” I’m all like: “I know.” - Gaining a Level
I like making lists.
It’s either Oscar or Whiskey:
“Duty called. I answered. We hung out. Duty is awesome!”
ISIC (I think):
“Your chest would look great. On fire. On my mantle.” (or something very similiar. Maybe “That’s a nice chest. It would look great on my mantle. And on fire.”)
Everything Orendi, most everything from Toby and ISIC, some S&A, WF, OM, the sentries (both story AND PVP). Here are my top five:
ISIC (not sure what activates it): “I’m not a gun; i’m SEVERAL guns. Duct-taped together. With legs; and a limitless capacity for hatred.”
Orendi (upon respawning), (in 20’s Boston voice): “Ya done screwed up, Orendi; but’cha got anotha chance!”
Toby (upon activating Core Discharge): “Knock-Knock, ■■■■■■■! Oh, god- SORRY FOR SWEARING!”
Toby (upon respawning): "Go on, call me cute; SEE WHAT HAPPENS!!’ -cute hiccuping noise-“Arragh, DAMN it”
Orendi: (I THINK upon reviving a teammate): -makes car revving noises, followed by a “brake-screech”- (in deep voice) “Get in, we’re gett’n the hell out of here.” -more revving noises-
The exchange between Kleese and Nova in the Algorithm where she says, “Kleese has got this you guys. I’m taking five.”
The “dibs” dialogue from Deande during Renegade.
Pretty much everything Nova says during Saboteur, but specifically the exchange between her and Deande that ends with “…and now I’m kind of freaked out.”
The exchange between Melka and Boldur about her horrible haircut in the Experiment.
Everything Nova says when she’s explaining the menus. “Dolla dolla bills.”
AH! I just wanted to post that one, you beat me to it. That’s Oscar Mike.
I also just started playing Toby and… It’s a bad character for me, because EVERYTIME he says something I’m like "Awwww… OH PVP RIGHT I GOTTA FOCUS"
Anyways, one of my fav Toby dialogues(I’ve only heard it once so it’s prolly not exactly what he says):
"If we survive this, I’m making us all matching uniforms!"
It’s so ridiculously cute.
Couple Ernest lines after having mastered him:
(these are both after respawning after a death)
For those who haven’t plsyed him, imsgibe a gruff drill sergeant type voice with a bit of a Southern drawl.
“The afterlife was boring as hell so I punched Death himself in the face and came back to fight with y’all!”
“I’m back after what was admittedly some good hustle by the bad guys.”
Toby when seeing Toby:
“Awwwww, 'he’s so cuuut- Oh dammit”
Toby when respawning:
“C’mon, call me cute! SEE WHAT HAPPENS! -Cute little laugh- Damn”
Oscar Mike, on respawn:
I’m back in businesses! I mean…business. Aw, s**t, I messed that up!
More Toby. After Killing Benny:
“WHO’S LAUGHING NOW BENEDICT?! ME! I AM! HA HA HA!” The laugh is so fake though ;p
He said this when I killed Ben, but not sure if it’s a Ben only dialogue:
"THAT’S FOR… Argg, I can’t even remember!"
Pretty funny because before I killed him Ben yelled “Let’s be besties, you and me!”
Benedict laughing after a kill and proceeds to make chicken noises always makes me laugh!!
Also Marquis calling ISIC his spirit animal 
Marquis rath kill (boredly) “so much for dying in combat eh, muahahaha”
Some of Nova’s amusing lines are when you are less than Command Rank 3 and you try to access the gear menu. She gets steadily more cranky until she refuses to speak to you and just repeats “access denied” or something similar. It’s touches like that which gave me a good first impression of the official release.
Ambra’s cackling never fails to amuse me.