This thread is all over the place!
Mk1- OG. It defined the GTI and no one can argue it.
My favourite GTI is the Mk2 (16V). Of course you may say that I'm biased because I own one, but what's not to love?! Wider flares, stock recaro seats, infamous red striping, 2-piece bbs wheels, 16v badging everywhere, 4-light grille with fog lights, and a whole lot of comfortable bells and whistles (minus the cupholders...) It's light, fun, easy to work on, and it's got that driver to car connection that gens like the the mk4 don't have. Also, the 16v was a great contender in its days versus the honda crx and mazda I believe.
The mk3 can also be a great gti if tastefully modified and maintained. I never got one simply because of the rust, wiring issues, and lack of good engine choice. But I have seen many great looking mk3's. Although they do look "fatter", the body lines make it to be a more modern mk2 which as I've stated, can look great, especially with the euro bumpers and flares.
In summary, a Mk3 needs more work to look good, or you can buy a mk2 and be happy .
Mk4- Similar to the mk3, but fatter, with less driver connection. Engine choice and is way better though, especially the 03+ 1.8T with the 6 Speed Transmission. 20th AE was my favourite, especially since they brought back the recaros (not as good looking as the mk2 recaros though ).
Mk5- Beautiful. Took a little while to get used to, but love it. The FSI sounds like a nightmare, but they made things right with the TSI. What's in a Mk5 you ask? 16V, with a turbo on it . Mk5 to me represents VW getting back on track with the GTI. The interior got revamped, and reminds me of the comfort/sportyness that the mk2 recaros have. Options are nice too, leather or jacky cloth, well made steering wheel and cluster, I think they got it right with the Mk5 and continued to progress after that.
Mk6- Took a little while to get used to like the Mk5, but love it even more! All the bugs have been ironed out and the facelift is looks more aggressive.
* Also noticed that there are alot of Mk6's on the roads these days *
Mk7- Can't wait to get a better job so that I can buy one! I see nothing wrong with it, well done VW! They finally realized that they needed to squeeze more power out of it in order to continue to compete.