Pokémon Gold/Pokémon Silver/Pokémon Crystal:
If the selected Pokémon only knows one move:
"That Pokémon knows only one move."
Pokémon Ruby/Pokémon Sapphire/Pokémon Emerald:
If the selected Pokémon only knows one move:
"<Pokémon> knows only one move so it can't be forgotten…"
In Emerald, if the move is Surf and the Pokémon is the last one that knows it:
"Hm! Your <Pokémon> doesn't seem willing to forget Surf.
Pokémon Fire Red/Pokémon Leaf Green:
If the selected Pokémon only knows one move:
"<Pokémon> seems to know only one move…".
Pokémon Colosseum:
If a Shadow Pokémon is selected:
"Hmmm... I'm sorry to disappoint you, but there's nothing that I can do for this Pokémon. Would you like me to make a different Pokémon forget a move or two?"
If the selected Pokémon only knows one move:
"<Nickname> seems to know only one move. Would you like me to make a different Pokémon forget a move or two?"
Pokémon XD: Game of Darkness:
If a Shadow Pokémon is selected:
"Hmm... I'm sorry, lad, but that Pokémon is too much for me to handle. Should I make a different Pokémon unlearn its moves?"
If the selected Pokémon only knows one move:
"Hmm... Your <nickname> seems to know only one move. I can't make it unlearn that. Should I make a different Pokémon unlearn its moves?
Pokémon Diamond/Pokémon Pearl/Pokémon Platinum/Pokémon HeartGold/Pokémon SoulSilver:
If the selected Pokémon only knows one move:
"That Pokémon knows only one move, so it can't be forgotten..." (Returns to Pokémon selection screen.)
Pokémon Black/Pokémon White/Pokémon Black 2/Pokémon White 2:
If the selected Pokémon only knows one move:
"That Pokémon knows only one move, so it can't be forgotten..." (Returns to Pokémon selection screen.)
Pokémon X/Pokémon Y:
If the selected Pokémon only knows one move:
"That Pokémon knows only one move, so it can't be forgotten..." (Returns to "Which Pokémon [...]?")
Pokémon Omega Ruby/Pokémon Alpha Sapphire:
If an Egg is selected:
"Hrmmp? I'm fairly certain that, um, Egg doesn't know any moves."
If the selected Pokémon only knows one move:
"Um, that Pokémon only knows one move. Hrmmp, I can't make it forget its only move..."
Pokémon Sun/Pokémon Moon/Pokémon Ultra Sun/Pokémon Ultra Moon:
If the selected Pokémon only knows one move:
"What's that, friend? That Pokémon only knows one move, you know?"
Pokémon Let's Go, Pikachu/Pokémon Let's Go, Eevee:
This part cannot be answered yet because (for some reason) no one has a list of the dialogue for the Move Deleter.