Explain your reasons. Bonus points for silly answers.
Arwen. Definitely Arwen. Any female with a sword named Hadhafang is aces in my book.
Dude. It’s fantasy, so:
ménage à quatre
Arwen is Galadriel’s granddaughter, so that’s just ooky.
They’re all hawt, but I’d probably be more comfortable with Eowyn. I’d always worry that Galadriel was peering into my soul, and Arwen would play the guilt card: “I gave up the best years of my life for you – thousands of them!”
Plus, of course, Galadriel is already married to a mighty elf lord. Yeah, he’s pretty passive in the book, but I bet he could be dangerous if riled.
Arwen. Her name is the easiest one to spell.
But “Galadriel” sounds prettier.
Are we talking about which one we like in general, or which one we’d date?
I deliberately left the question open-ended. You can answer it however you wish.
Galadriel. I like older women.
It’s okay, as an aficionado of identical twin sex videos, I’m aware of where the line is and can direct them accordingly.
Eowyn.
She’d have my back in a fight.
JRRT’s Arwen never seemed much more than decorative (despite her ancestress Luthien kicking Sauron’s butt), and Galadriel is just a bit too worldy-wise for my taste. All that time sitting at the feet of Manwë and Varda, y’know.
I’m pretty sure that if Gimli & Eomer understood what you were talking about, they would kill you were you sit.
They’re ELVES. They don’t quite have the same view of these things as we do.
How about Goldberry, can you throw her in too?
Write-in vote for Melian. She protects the land with her magical girdle. I always pictured some French Quarter cancan girl chasing the orcs away with her divine sassiness.
That’s simple madness. If Galadriel is intimidating, Melian must be intimindating-cubed. At a minimum.
She can intimidate you with her mind!
Eowyn, because cuckholding Faramir doesn’t strike me as being nearly as dangerous as doing so to Aragorn or Celeborn. 
What if I weren’t sitting?
I left out an H, obviously.
But actually sitting would be the best option. I don’t think either of them would kill you without giving you the chance to defend youself, so if you stayed in your chair they’d forbear.
Well, Eomer might.
I’d still be taller than Gimli.