Singlespeed & Fixed Gear - Best way to remove grease and other crap from chain/chainring/cogs/ etc?

What is the best way to get all that black ugly grease and crap off? I want my bike to look clean and brand new. Is it best to use wd-40? Simple green? What do you guys use?

Simple green and a shop rag if just a dry shop rag doesn't do the trick. If it's stubborn let the simple green sit for a few moments and then wipe.

If it get really bad you can remove the chain and give it an oil bath. This will remove more of the gunk than just wiping the chain with a rag. I usually just take the chain off, give it a good lubing and wipe off the dirt with a rag.

Mineral spirits. Take everything off and dump 'em into a parts wash tub with mineral spirits. Slosh it all around, maybe use a scrubby brush for the big chunks, and then pour off the spirits to a catch container with a lid. You can let them settle, pour off the clean part and use it again and again.
Refill the wash tub and re-slosh your parts until they rinse clean. Dry 'em off (I use a fan to dry the chain quickly) reinstall and re-lube.

Take your WD-40, remove the lid of your trash can and insert the WD-40. Replace trash can lid.
Seriously, with the exception of unsticking rusted bolts on my Jeep I've never found much point in having it around and it invariably gets used by someone for the wrong purpose.

If it get really bad you can remove the chain and give it an oil bath. This will remove more of the gunk than just wiping the chain with a rag. I usually just take the chain off, give it a good lubing and wipe off the dirt with a rag.

Could I just bathe the chain in simple green?

spray down with simple green and the scub down dawn dish soap in a bucket of warm water ... rinse gently around bearings and such use cheap dollar store scrub brushes viola ... park chain cyclone with chain bright or a citrus based solvent is a awesome way to clean the chain on the bike ... I use all 3 for every bike wash.

I'll add to what I said here, that was a full bike cleaning I was describing. to clean the chain a sparkly as your a shooting for get a chain cleaning tool such as the park chain cyclone I mentioned it works awesome, the park chain brite works great and it's biodegradable. Pedro's makes similar products.

Gasoline

kerosene
mineral spirits
what have you

-avoid turpentine and other such thinners that can be very harmful to you