Hmmm, They're all good after say the MK VIb, but I'd say the ultimate was the MKXXI Very sexy with the bubble canopy, although I'm somewhat partial to the 'malcolm hood' type of canopy.
The clippped wing was very cool also. Hmm, so many I dont think there was really a bad Spitfire... But Kinda like them after the Mk IV. Seems like they kinda evolved at that point. Not that I'd turn down any...if someone were to want to donate one to me.
It's popularly held that 'the' definitive Spitfire was the Mk.IX, as it easily matched anything the Germans could throw at it at the time, and was second only to the Mk.V in terms of numbers produced. Of course, things only got better from there.
In terms of outward appearance, I like the looks of most of the later marks, with the tapered rudder and twin radiator baths. Malcolm hood or teardrop canopies look good, IMO.