If I could really only have one, and I mean at gunpoint, sorry it would have to be WinStep.

Windowblinds!

This may sound odd, but Photoshop.   It's all about the wallpaper for me first!

DesktopX.

Winstep Xtreme

Pencil.

Windowblinds..

tho Litestep will always hold a special place..

For using: WindowBlinds

For skinning: RightClick

Definitely Windowblinds. Winstep is a close second.

OK, I change my mind a bit.

For using...WindowBlinds.

For skinning....DesktopX.

Iconpackager and Rainmeter 

Ditto...

ObjectDock by necessity; WindowBlinds to soothe my eyes. (And DesktopX is almost necessary, too). See---that's only one, just like you asked.

WindowBlinds

I couldn't live without Windowblinds.

I'd be over run with desktop icons again without Fences, but I really love Cursor FX......can I have three?