This of course would span all generations. Basically a game that started out decent, maybe showed some promise, but never really left much of an impression... and then its follow-up came out only to BLOW YOU AWAY.
I've been thinking along the lines of Mega Man 2, possibly Mass Effect 2, although I did miss SOME of the RPG elements. Assassins Creed 2 and GTA 3 would be good choices too, although I don't really know if I could say GTA 3 was an IMPROVEMENT, being that the previous games were already very solid and enjoyable, the third was really more of an exciting new direction. Then there's of course Uncharted 2... that game might be the one for me. What's yours?
Surely it is Doomdark's Revenge. The size and scope of it was staggering enough but the degree of improved strategy over the first game was what kept me playing this monster for nearly a week before I had to stop myself.
The first game broke the mold in innovation and pushed everything to the limit that the very idea alone sounded impossible. The sequel is crazy. That an idea so vast and also supported multi player campaign with more than double the map size of the first game with vast legions of the major races controlling five different realms its a wonder that it all fitted snugly into less than 48 kilobytes of memory. Doomdark's Revenge rocked. Only thing I didn't like was the cruel difficulty level. Like all games that were made in that era, this was the only hand holding you would get...the part where it takes you all the way back to the start after 18 hours of play and says 'Ok, let's do this again...from the top'. As a sequel, it was and remains unrivaled in gaming history. If I am not mistaken, it was written by just one guy. But this one guy is like an entire dev team with the smarts to boot.