Hard one to answer - generally if it is made by Park it will be great at doing what it was intended for so whichever tool I bought last will probably be my favourite; repair stand makes the job so much easier so that is also on the long list; Hypercracker, haven't used it in years but it still has pride of place in my toolbox; think it is a toss up between a Minoura chain spring, little springy thing which holds your chain whilst you connect the two ends together (best fiver I ever spent) or it could be a Tacx break setting tool - seats V brakes, cantis and road calipers perfectly.
Oh I can't decide