"Best" is relative. Budget best? Performance best (stock)?
I have owned 4 Z33s from earliest 2003 model to '08 Nismo (plus '12 Nismo Z34). Best? My Z34. But it had better be, it cost $45,000 and was the most advanced and most powerful Z at the time (and honestly, until the Z34 Series II finally gets past the marketing hype and shows up at the dealers and on the road, my Nismo IS still the fastest). That all said, the most "desirable" are the more powerful and higher revving 2007-08 HR equipped cars.
BUT, as dkmura said, any reasonable miles, well maintained Z with records of almost any vintage is worth having. Case in point: 5 modern Zs and I retained my oldest one as my keeper. It has the usual foibles that the '03 is known for but in mine, they're all managed or fixed, e.g. 3rd gear gronk, alignment, respectively.