What’s your go-to phrase for sounding polite but still being a little sarcastic?

Thank you for your input.

I would suggest… I hear what you say. I‘m keeping track of my appointments in the digital calendar.

I'll deal with it momentarily. Whether that means soon, or briefly, I leave up to their imagination.

“Bless your heart” in the US south can be a back-handed compliment with nuanced meanings.

Or "You haven't really thought this through at all, have you." Honestly "what problem are we trying to solve" is a great re-direct, especially if they didn't start there.

“Duly noted”. I’ve only used it a handful of times but that’s my professional way of saying “fuck you”, as I’ve only ever used it towards clients being extremely condescending and rude towards me.

'gee, really?'

"Thanks for your feedback."

“If you say so.” 

Thanks for your feedback.

I use the word ‘ambitious’ to mean that the suggestion or idea is completely unrealistic and not reasonably practicable

As per my previous email…

Your question will be answered in the order it was received.

With all due respect,

I’m a software developer and something I say at least 3 times a week is “what problem are we/you trying to solve?” It often means “I don’t think you asked the right questions” or “I think you may be doing the wrong thing already”

"You...you go do that."

You do you.

As previously mentioned/suggested/explained (to you idiot!)

As you already know… (so don't ask me again!!!)

Credit where credit due. Ie not here.

Thank you in advance for your understanding and cooperation.

I understand where you’re coming from, but unfortunately this isn’t up for debate.

I can’t tell you how sorry I am that you feel that way.

“I love that for you”

Quelle surprise 

“There you go”

Manager at a hotel: "How about we try (blank)instead" meaning "what you are doing is absolutely wrong, stop doing it we're doing it this way"

I’ll take that under advisement

As I’ve *already* said…

Bless your heart...

Bless your heart.

I hope you know

Oh, don’t mind me!

Warmest regards

Yes, we’ll have our best people on it as soon as possible.

Actually….

Great. Other ideas?

No, please go right ahead. Don't let common sense or manners stop you.