…fuck this. I’m turning gay.

Celestia is moderately risky - Generally Twi will probably be elsewhere and easy to avoid but who knows what the heck her spells would do and if she would reverse any of them. Gotta pick her just because she’s the best of both.  
Luna is the riskiest of the bunch (but damn is she worth it!) - She lives in the same palace as Celestia and the ROYAL CANTERLOT VOICE is a dead give-away so getting caught probably won’t take long. Having no free will is cruel. Having no free will for life is just pure evil!  
Cadence is an obvious risk seeing that Shining is her personal guard AND husband. Still, only having to undergo physical training and a little sparring match (the only real danger being his magic) is probably a positive in the long run. If I liked Cadence more I’d pick her safe option.

the downsides…are worth it..Celestia

Celestia, and Twilight will never know

I choose neither, as I am already in a happy relationship with Rarity the unicorn.

I think I’ll stay a virgin.

Celestia, no doubt about it.

I have a horse game called Lucinda Green’s Equestrian Challange xDD

@Corrderio  
Depends on what you consider “degrading”. If degrading is “I’ll remember it in the morning”, then yes, simply because you wouldn’t.

… S&M with twilight? well ok i’ll pick celestia for that

I’d go with Luna.
 
Celestia wouldn’t make me do anything too degrading right? Right?

Princess Cadance

Cadence,she sexy

Luna, because nighttime rules. I’m also have ninia skills so Celestia can’t get me. ;)

Celestia… such a beautiful mare…

“My apologies, Your Highness, but as a commoner it would be inappropriate for me to endanger the image of a Princess.”
 
(Yeah, they’d have to arrange things so it was legal, moral, and didn’t have to be secret.)

Celestia. Because, well, I love Celestia.  
That and becoming a mutated magical monster might be fun, actually.

Luna all the way.

I’d go with Celestia. However, the “affair” wouldn’t be anything more severe than talking about the history of the world and maybe some mutual wing preening.

Tough one. But Celestia wins. I would hate to be a servant, and I’m not fit for soldier training. But with Twilight, I can learn to DODGE!

Luna.  
I don’ t care.

runs away, screaming.

@anonymouspegasus  
(I don’t have a ponysona)

Buck it, I’m coming back to elaborate, this is fun.
 
You’ve already got my REASONS for picking Celestia. Now, I’d keep the interaction at a minimum, contained entirely or at least almost entirely within the castle, a place Twilight doesn’t tend to spend her time anymore. I’d avoid interaction with Celestia in public as much as possible and make damn sure to get to know Twilight at the same time just in case. Build up a rapport, y’know?
 
Maybe get to know her friends, too. Maybe they’ll defend me if I do get caught.

Celestia it is. Twilight’s not a particularly cruel pony, even given this situation.
 
Besides, I can’t fight worth spit and I’d prefer to take a piss whenever I want to and not whenever somepony else remembers I need to.