What is the most light-weight symmetric cipher thats still usefull? I need a very light encryption scheme which costs almost no performance during encryption (while decryption may be slow).

I found 5 algorithms: RC4, DES, LED, PRESENT, and Piccolo. 

But how do I pick one that's faster than AES on my platform?
Salsa/ChaCha and the other eSTREAM winners are likely to be the "fastest but still secure" options today.

Don't forget authentication of course. Reduced-round ChaCha/Poly1305 is likely to be the fastest software-only option, due to tuned implementations in the libsodium and NaCl libraries.

UPDATED: The following slide deck has good info on state of the art lightweight block ciphers vs. legacy (along with source code links). Considers code size, RAM, and cycles/byte metrics for many algorthims). 

Stream ciphers such as those from eSTREAM may be a better option than block ciphers. Even choosing RC4 and hardening it by dropping the first 512 bytes may be a good solution; code size doesn't get much smaller than RC4.