Think about it
 
If you tickled Derpy, she’d probably kick you accidentally pretty hard

I would do Cadance, but Shining Armor would punt me into next week.

Derpy, she’s the cutest.

ALL.

The Derpster. Totally rad.

Spike.
 
I know if I tried to tickle Derpy it could only result in me getting injured, since this is Derpy we’re talking about. “I just don’t know what went wrong” is what she’ll say on my funeral.  
If I tickled Cadance I’d probably end up hurt by her gargantuan horn or legs, or I’d end up imprisoned for tickling a princess (or both, because it’s probably considered creepy as hell)  
Spike has claws, but Apple Bloom managed to tickle him without getting herself killed.
 
I think I’ll go with Spike, I mean he’s a baby with an adorable laugh.

Derpy.

Spike has sharp claws, a tail that can dig through dirt, and his spines can cut through things too. If I tickle him I may die.
 
Cadence is huge with a horn that’s at least a foot long, she could easily stab me accidentally and kill me.
 
For safety’s sake I choose Derpy.

derpy!

Cadance.

Tickle fight with cadence? Why not?

Derpy!

Derpy

Derpy.

@Background Pony  
Yes I would like to get a restraining order please

Spike, as mammals are abhorrent!

i would choked cadance for not inviting spike but i’ll tickle derpy

I would tickle all three.
 
But only of they ask for it.

Cadance, out of those three.

Why not all three?
 
But in order:  
Derpy  
Spike  
Cadance

Who the hell would wanna tickle spike?  
He’s not even a pony.

Derppy!!!