I'd grap a towel and cover myself completely with it.

I would just continue getting dressed while yelling at them for being a douche!

Grab the damn towel and wrap it around me tight enough to cause lines. I hate it when someone other then my husband seeing me naked, it's already bad enough I can't be anywhere in my apartment where there's a window to peek in through.
If someone walk in and saw me naked, I would:
1. Wonder why they're not leaving.
2. Kick their ass the second I can, and many people know I will.
Then I get dress.

I would grab whatever was closest to me

I know screaming isn't necessary, but I'd scream anyway.

I'd reach for a towel and tell them to get out.

Full disclosure here...am I really the only one with enough exhibitionist in him to not be bothered by being seen naked by someone of the opposite sex? If they want to stand and look, let them. Not saying I'd do anything, I'm married, but it wouldn't bother me enough to worry about it. Maybe it's a guy thing.

I'd grab that nearby towel and tightly secure it around me.

My snarky responses will be going faster than my hand for the nearest material object -

"Let me introduce you to an alien custom from Betelguese called 'knocking'."
"Aren't you wishing you were struck blind before you came in?"
"Yes, is there something you need in here or do you not have respect for privacy?"
"I hope I haven't disappointed you too much, but my beach-body suit is at the cleaners today."

Or if I can see that it's obvious he's blind: "Sorry, but this isn't the men's room. Do you need any assistance?"