Anything with a bit of engineering skill and a deep and forgiving pocket.
There is a twin engined mk1 with 2 audi engines in it. Basically put two engines in a mk2 car and you are on your way to fame and reputation in the dub scene
Most powerful i have seen is a 1.8T, they get tuned to silly levels but then they are very liable to be blown up. Pretty cheap way for big power gains, also a VR6 conversion is a popular one. supercharged, turbo'd or both.
Really it is entirely up to you how individual you wish to be as anything is possible with the right application of mechanics and engineering ability.
