Well the title pretty much says it, and I'm sure there has been a poll like this in the past, but I didn't see any that's still active so I think this is fine.
Here are some from old episodes I've recently watched (or just remember because they are so awesome). And as I will do in my examples, feel free to paraphrase the quotes, they don't have to be exact as long as the joke/point is clear.
From "Gotta Catch Ya Later":
Jessie or James: How could those goons lose so easily? What a trio of losers!
Meowth: Yeah, now let's show them how real trio of losers is supposed to act!
Not sure what episode, or even season (might have been orange islands or original though) this is from:
Meowth: (sigh) if I had a nickel for every time those two ran off without a plan, I'd be richer than the boss!
From "There's no place like Hoenn!":
May: Torchic, use amber!
From the manaphy movie:
Jessie and James: woah, diamonds, and pearls!
Meowth: let's get through this season first!
From "Now that's Flower Power"
Jessie: Wow, this couldn't have worked better if we planned it
James: If we planned it it wouldn't have worked out at all!
And while I just can't remember the actual line, there was this really funny comment Jessie made in the episode "Hot springing a Leak"...if anyone knows it feel free to remind me/share it lol.