The Last Autobot was a guardian that Primus prepared, against the day that he would no longer be able to guide his creations.
There are Decepticon legends of an "Ultimate Warrior", but it is believed that they are corrupted accounts of the Last Autobot. He is apparently also sometimes referred to as the Heart Of Cybertron.
His powers were used to stabilise and recreate Cybertron after it started tearing itself apart due to Primus's "demise" in the battle against Unicron. He was also responsible for resurrecting Optimus Prime back into a robotic body.
The Last Autobot seems to be unique to the original G1 comic, but his face bears a strong resemblence to the Autobot insignia, and his alt-mode is an ancient Cybertronian spaceship. In these respects, he bears a striking resemblence to Vector Prime in Transformers Cybertron/Galaxy Force.