iOS 4.2.1 on iPhone 3G
After being fed up with 4.0 on my 3G, I downgraded the firmware back to 3.1.3. iOS 4.0.1 was supposed to fix the performance issues on the 3G but I wasn't so sure. So, I waited until 4.2.1. Jailbreaking was easy using redsn0w (the part that I always forget to do it upgrade the firmware on my iPhone first then jailbreak).