I felt that in a vacuum, GT6 was better over GT5. Better suspension model, more racecars, better tracks, etc. Perhaps the main thing I felt it was missing was proper endurance races, it was quite the let down to discover there would never be anything better, within career mode, than 24 minute races.
I can never get over how often both GT5 and GT6 would crash, never have I played any other games that came close to crashing nearly as much.
Back to how I felt GT6 was great in a vacuum; outside of that vacuum was Assetto Corsa and the impossible-to-play-on-controller Project Cars. While I easily put over 100hrs in to Assetto Corsa, I hardly put more than 10hrs in to Project Cars....yet I probably still got more enjoyment out of Project Cars than the majority of the 120hrs+ I put in to GT6. I didn't get in to AC and PC until late in to the game, but once I did, my PS3 started getting quite dusty.
I suppose with the bad aftertaste GT6 left in my mouth, it could be ranked last. I know with high certainty I won't be springing for GT Sport, or even a PS4 any time soon. I may very well never buy another GT game again if gameplay remains similar. It's an unfortunate reality, considering I got so much out of it in the beginning. But between finding much better games that simulate on another level, and experiencing multiple trackdays in my little 1.6 Miata, I found the GT games to be lacking in what I needed out of my racing games.
Still, I tend to only pop in GT5 for the F1 cars, or some of the tracks. I still tend to prefer GT6 over GT5, but I can sort of understand the final decision to make it last.
I was surprised GT3 ranked so highly, it seemed like GT4 was the clear winner. I only got to experience GT3 when I was very young, and even had a wheel for a short while. I enjoyed faux-go-karts, and definitely determined I already loved cars. But for some reason, the realistic and unforgiving gameplay, at a time I was still 10 or so years before even getting my license to drive, was too much. So I didn't play GT3 for long. But looking back on it, it seemed like it was missing a lot. I can't say much though, I never did buy GT4. But all the videos and reviews I watched made me think I would've loved it over GT3.