In terms of real-existing sci-fi, I'd have to go with either a TARDIS, Unicron, or Megas.
But since you also state that made-up transport is possible, lemme tell you about an evil creation of mine...
It is a large-ish monster of a mech, more or less draconic in nature. Think of a dragon that is semi-humanoid. It walks on two legs and has two arms, but it has a saurian gait and two tails. Clawed feet, clawed hands, and the tails are long and ridged like a dragon's. It's a black-and-red mech and it was designed for destruction. Ten long-and-retractable tendril-like conduit-limbs end in an open spout that spews out a crimson gas or energy bursts. Its back has a tumorous growth which engines seem to grow out of for its flight purposes.
The machine has a generator which is always appear to be in overload, delivering power to all the systems at such a rate that the massive energy bursts it gives off can actually damage it temporarily. It is a regenerative machine, its entire body having altered and adapted into this form from another to handle the stress of the power it was put in command of. The machine is malevolent, capable of incredible deeds. It needs a pilot to stabilize, though, to coordinate energy properly. It lacks imagination without one.