This being Bogleech, there's a high chance we already have this (and if we don't I'm baffled as to why), but just in case, here it is. None of these pictures are mine, by the way.
I guess I'll start. Inexplicably, one of my favorite youkai (I'm gonna continue to spell it that way, deal with it) is Wanyudo, a flaming man-faced wheel which eternally rolls to the underworld and back, bringing with him the souls of all he encounters. I'm not sure as to why, but this one was always really, really cool to me. Something resembling it appeared in Okamiden in the form of Fire Eyes, although instead of flaming wheels with faces they were flaming tops with eyes.

I also like the Heikegani, crabs said to be the souls of a group of suicidal samurai. They actually exist, although the stories are much more fanciful than their mundane, normal-crab lives. I can't find many other good pictures, so here's the Heike from a japanese SNES game.

KAMAITACHI! Weasels with sickles for hands. It's not hard to see why I like these things.

Last but not least, I give you Hyakume, a hundred-eyed sometimes vampiric creature that only comes out at night. He's said to protect a shrine somewhere and his eyes can detach and fly around. Cool.

What are your favorite/least favorite Youkai?

Favorite undead thing ever and it happens to be a youkai. I particularly like how mysterious that entry of it is. It doesn't do anything that we know of, other than creep people out. It is neither benevolent nor malevolent, it simply exists. Western culture doesn't do this in mythology and folklore enough.

oddly enough, most of my favorite yo(u)kai are the ones that seem normal at first, like the nukekubi, who appears human by day, but flies around as a severed head at night, and the rokurokubi, a woman with an extendable neck.

I love the Bakekujira as well. I know I have an annoying tendency to connect everything that exists to Lovecraft in some way, but this one's justified. He was big on the 'no good or evil' and this really reminds me of his stories.

Youkai are WONDERFUL. <3 I take it you've heard of Kitaro?

I'm also a fan of the "Hey I'm just an ordinary human OH WAIT NO I'M NOT" variety of youkai/yokai/yookeye/Those Weird Things.

I also quite like Tsukumogami in general, if only for the idea that any common item, even parts of a house itself, might become a monster if it's old enough.

Youkai confound me sometimes.
Half of them disguise themselves as lost children or fair maidens, then eat anyone who shows them kindness.

The other half are just normal beings who'll get mad if you deny to help them.

So basically the maiden you see on the road miles from civilization asking you for a ride home is probably not human, but if you decide to be nice and help her out, she'll probably suck your soul out of your ass the moment you turn your back.
But if you ignore her, she'll turn into a giant firebreathing spider and eat you because you're being a douchebag.

You can't win with Youkai!
Then again, if by some miracle you manage to be nice to the decent ones and survive the assholes, you JUST might have a new firebreathing spider woman for a best friend, and can go on adventures.

Speaking of whom, my favorite youkai is almost definitely the jorogumo: http://en.wikipedia.org/wiki/Jor%C5%8Dgumo

The classical spider-woman.
Who also inexplicably breathes fire in some versions.

Speaking of asses, the Shirime is another favorite of mine. Anyone who's read articles by scythe on Cracked knows what I'm talking about, but if you haven't, well....

Welcome back, Gregole!

Yeah, but thats what makes the youkai fun! Its like a lottery, except all the coins explode into acid!

Assuming you're talking about GeGeGe no Kitaro, yeah, I've heard of it. The premise is a bit weird, but cool nonetheless. Of note is that You played a Rokurokubi in the 2007 live-action film. No, really. The actress' name is You.

Have any of you seen the Yokai Monsters film series? Rather than featuring just one monster, they feature dozens upon dozens of monsters in one scene.

My most favorite youkai has to be the umibozu. I'd love to see Godzilla fight one. Or a bunch.

I know I've seen part of one once. It was the part when they blinded the bad guy (his name was Daemon or something).
I like Umibozu as well. The commonly vague descriptions and depictions of it allow lots of creativity when drawing or writing about it.

The name umibozu literally meaning "sea Buddhist monk," I can't but be reminded of the sea bishops and whatnot of Western lore.

I've recently found out about a couple less popular (but no less deserving) Youkai that I thought were cool.
Amanozako, a youkai deity with long ears and nose, jaws that could chew a blade ragged, and is capable of flying for thousands of miles.
The Namahage/Anmo are demonic bogeyman type creatures that resemble small Oni. Tradition has men dress up as them and wander around town admonishing poorly behaved children.
Enenra, a creature made out of smoke. That's all the info I could find, but I think the idea of a living gas is pretty neat.
The Gashadokuro is pretty well-known, but I just wanted to mention it for probably being one of the scariest, most intimidating Youkai ever. Most of these are mischeivous little sprites, right? Then Gashadokuro comes along and a giant skeleton of unburied bones and he's one of the few genuinely malevolent Youkai.
Finally, there's Isonade, a predatorial fish-man that has a barbed tail which he uses to ensnare sailors and devour them.

ITSUMAAAADEEEEEEN

Sea Bishop VS Umibozu

Couldn't help it.

GO SEA BISHOP GO

My money's on the Umibozi.

This need to be made into a SciFi Channel movie.

Has Syfy ever made an original film featuring youkai? I see nothing of the sort, sadly.

They haven't yet. Yokai/Youkai/Spooky Motherfuckers are gradually getting more and more well-known in the states via anime and manga, so I definitely think it's only a matter of time.

Huh. Apparently I started making this game last year called 'Cuke the Kappa.' What a weird name. I just found it again now. All I had done was the player character, so yesterday I added in a basic enemy in the form of a Wanyudo and an attack for the Kappa. Here's a screenshot of what's done.

I like what I see. Do you plan on continuing this project?

And uh... Do you plan on throwing in a tanuki or a gashadokuro?

Probably. yesterday I had the idea of a Tanuki that used its testicles as weapons and would be the first boss. And there's no way I could leave out a Gashadokuro.

In hellboy sword of storms there a part were hell-boy has to fight a whole haunted graveyard of random yokai. The movie also has him fighting a spider women, a kappa, as well has a few artifact spirits. but I cant find any good pics of thoughts.

That whole movie is full of yokai. I liked it much more than the other animated Hellboy movie, partially because of the yokai.

HOLY SHIT THAT IS THE BEST GASHADOKURO INTERPETATION I'VE SEEN SINCE OKAMI.

I've always liked Ittan-Momen, if only because the idea of a strip of cloth just flying up and smothering someone is hilarious to me. Especially if it's a completely failed smothering.

Also, one thing of note about the scythe-weasel from the first post is that, if I recall, it will fly up to someone, cut them up horribly, instantly heal all the resulting wounds, and fly away. It basically just likes freaking people out.

I've also heard the instantly healing version, but it then repeats what it did before, causing an endless cycle of being cut, then forcibly healed.