I've learnt so many versions of the Two-Step, What should I be using as my basic step, and what ballroom dance closely resembles the two-step so that I can use moves from it?

By Two Step, I assume you mean a Country Two-Step, as opposed to a Nightclub 2 Step (which is also danced in the Country and Western World). There are many social variations of Country Two Step (Texas Two Step, etc), but here is a basic descripton of a Two Step you would dance in a more formal environment , like in the UCWDC dance circuit:

1) Step forward onto the left foot for 1 count of music (QUICK).
2) Step forward onto the right foot for 1 count of music (QUICK).
3) Step forward onto the left foot and hold (2 counts of music, SLOW).
4) Step forward onto the right foot and hold (2 counts of music, SLOW).

- In terms of Ballroom dances, Foxtrot is the closest to Two Step, however they have two completely styles. Moves from one don't translate very well into the other. Foxtrot is a slower, smoother flowing dance. Two Step is a faster rhythm type dance with LOTS of fast turns.
Hope this helps!