Gran Turismo 3 A Spec vs Gran Turismo 5 - This comparison's a little old but GT3 was so much more fun to play than GT5. The range of cars, mods and single player campaign made GT3 feel fantastic and well worth £40 at the time, GT5 had far less in comparison and was promptly sold later that week.
This one especially for me. I watched a YouTube video recently comparing tracks on the game and how they've come on over the years, and I noticed the way the graphics, sound and engine had developed over the years. I think GT4 was the high point of the series. Things were continuing to improve game by game - although some aspects in 4 for example were copied straight from 3 (like 6 from 5, premium and normal cars..) - and since I feel the time between games seems to get longer each time, any upgrades seem less and less of a step forward.