The Arsenal of Freedom
This is probably my favorite episode so far, and surely the most exciting. It has the rare distinction of making effective use of almost every member of the regular cast (except Wesley) and even the Enterprise (taking advantage of the rarely seen saucer separation). It's a good story that would have been right at home on TOS. In particular there are similarities to The Doomsday Machine and That Which Survives, but it's original enough not to be a retread.