Q: In Nasu's work (Kara no Kyoukai, Tsukihime, Fate, DDD) Who's the top three strongest characters?

A: Magical Amber, Neco Arc, and Tiger. Well, not really.
Seriously, it's Arcueid, "Ryougi Shiki", and primordial demons.(Not counting Servants)

Comptiq 2007-05 issue - Fate Dojo Q & A