Uncomfortable truth: The game was shite when there were only 151 Pokemon.
Uncomfortable truth, indeed. Except that at the time, Red and Blue were masterpieces. At least in my opinion anyways. But I was kid, so what the fuck did I know? They were one of the few games to receive a perfect score from IGN, which used to mean a lot more in the 90's.
As somebody who's played each of the games (I've only dabbled in Gen V, but it's alright), it's hard to go back and play through Gen I again, unless you're doing it with the GBA remakes. It's nostalgic, no doubt about it, but the developers made the game so much better in the next few generations. THe basic combat stayed roughly the same, but all the little ins-and-outs that were added over the years are why I can STILL find the games enjoyable. Simple mechanics like "registering" an item or using the Running Shoes. Breeding Pokemon and having more than one guy in the Daycare.
In my opinion, the game is at its best in Generation II. Pokemon Crystal was, until I picked up Heartgold, my favorite Pokemon game. It might still be. Not only do you get this brand new story, but when you're done completing it, you get to play through the old story again! How cool is that? It's not that impressive now, but imagine playing Gen II again for the first time. Both legendary birds were badass, plus three legendary dogs? Awesome. The introduction of Dark pokemon, two more Eevee evolutions, an expanded move set, the aforementioned breeding mechanic, etc. Crystal was so damn good, because you start the game and catch a wild Phanpy from the very beginning. Kind of broken, but I'll take it... I remember the first time I caught Red Gyarados, it was the coolest thing. Shortly followed by realizing it was only the second coolest thing, because mother-fucking LANCE enlisted your help to beat the hell out of Team Rocket. I felt like a rock star.