Sonic the Hedgehog
The first ever Sonic game isn't terrible. In fact, it's still really good. However, it is the weakest of the three (I will consider Sonic 3 & Knuckles as one game). Green Hill Zone was a highlight, but there's a few levels (Labyrinth, Marble Garden off the top of my head) that don't quite reach that high again. Still, if you're going to play the Genesis/Mega Drive Sonic games, you might as well play this, it is still pretty good and you have to remember WHEN it was released, it would have been damn groundbreaking.
Sonic the Hedgehog 2
Sonic 2 is great, by this point they knew they had a successful formula, they knew what tweaks to make and how to play to their strengths. It even added a two-player element with the addition of Tails so your friends could join in. In essence, a perfect sequel; it took what the first game did well, made it better and then added more on top. With retrospect, it is essentially the middle-man between the first Sonic and the next one...
Sonic the Hedgehog 3 & Knuckles
As two separate games, they're great. Together they make the ultimate Sonic game, more good levels than bad, some great songs and the biggest levels of the series thus far. What can I say, it's excellent and if you like Sonic even a little you owe yourself to play this version.
Overall, the Sonic series has a little place in my heart. The first video game I remember playing was Sonic the Hedgehog (alas, the 8-bit version for the Master System). To finally be able to say I've beaten these games gives almost a sense of closure, after all I doubt I would be the gamer I am today without having grown up playing (8-bit) Sonic games. This won't be the end for Sonic on Super Game Fighter however. There is still potentially all the games in the Sonic Mega Collection Plus, plus Sonic Adventure, Sonic Heroes (ugh) and Sonic Generations, so hopefully we won't be seeing the end of our favourite Blue Hedgehog.
I will start posting Crash Bandicoot stuff next week, probably starting Tuesday when I have an actual proper good internet connection. I'll see you then!