What's the most useful part (in your opinion) for your creativity in building stuff in KSP?

[Azimech]

I wouldn't be able to decide. It all depends on what I'm building.

[kerbiloid]

Struts. The more of them - the stronger rocket.

[Rocket In My Pocket]

http://wiki.kerbalspaceprogram.com/wiki/Structural_Fuselage

Because the hole through the middle is actually real and usable! (Unlike every other part with one.)

You can do a lot of fun/creative things with this simple part.

[Fenris]

That's a tough one. I mentally went through my list of favorite parts to find my most useful one; fuel lines, struts, radial and stack decouplers, solar panels, and about 4 of my most used engines. Then I remembered SAS. Maybe one day I will be a good enough pilot/designer to get away from using them, but for now they go one every single rocket I create.

[LoSBoL]

For me, it all starts with a parachute