Definitely Underfalls
It combines my two favorite fandoms!
How great is that?!??!!!

All the fanart are great too :3

My personal favorite ↧

I'm actually writing a fanfic for Underfalls so hopefully I can get that out soon (I'm almost done with the prologue ;3)

*randomly searches Underfalls on YouTube*

WAIT

[A gif or video was added here in a newer version of Wattpad. Update now to see.]

IS THIS AN ACTUALLY THING
I NEED TO KNOW
I WILL FREAKING BUY THIS IS ITS REAL

ANYWAY

Some of my other favorites are

Portale (I love the name :3)

Undertale-Steven Universe (idk what the name is)

Wait
Is Over the Garden Wall and Undertale a thing
*googles it*
HOLY SHOOT IT IS

(WAIT. WHAT IF SANS'S PET ROCK WAS ORIGINALLY GREG'S
BUT HE GAVE IT TO SANS O-o)

AHHH I FOUND THE FULL COMIC

That's  now my head cannon for Sans's pet rock

I'm so happy right now

This AU might even pass up Underfalls

*continues to look at crossover pictures*

OMG

*SCREEEEEECHES*

I just found out
Bill x Mettaton (MettaBill)
Is an actual thing XDD

Welp I'm gonna have to end it here
I would do more but Wattpad only allows 20 images in a chapter

I have a lot more crossovers but I'd be here all day making this chapter

So
what's your favorite crossovers?

Here have this too