The good thing with Spyro games (at least, for the PS ones :p) was that they got better with every new one. The original "Spyro the Dragon" was terrific, it had its own formula, the plot was good (albeit, a bit straightforward, but that was to be expected) and the gameplay was good (simple, but effective). In a sense, it worked.
Spyro 2 brought a slightly deeper plot, but it stayed true to its roots. One of the better things that was added to the games was dialogue and voice acting, which was nicely implemented. However, the plot was still rather simple, but it didn't destroy anything that Spyro already got going.
Spyro 3 was the ultimate pinnacle of the series (considering that it went somewhat downhill afterwards). It brought in new playable characters, returning characters, and refreshing new levels. One thing I neglected to mention was the difficulty level. Spyro 3 was the hardest Spyro game of any PSone title (mostly having to do with the new playable characters). Tbh, I haven't even finished the third game, because some of the new characters were a bit hard to control (Agent 9, specifically).
The thing about the new Spyro games (at least, for the PS2, since I haven't played the GBA instalments) is that they try a bit too hard, or not hard enough. 'Enter the Dragonfly' was horrible. Glitches and other technical issues (such as loading times) aside, it was very much, a recycling of the old games. Dragonfly hunting was a pain (especially when you succeed, then strut at this latest achievement, and accidentally breathing bubbles on the enemy by accident while they pummel you to submission) and the difficulty was immensely haxed. Ripto, for one, would be very easy, if it wasn't for the fact that one hit from his attacks meant an instant OHKO as you tried to get back up before you were hit again. The story was recycled (essentially just Spyro 2 all over again, *except* it's not in Avalar!!!!) and the game was, essentially, just a giant version of one of the old Spyro worlds (think Autumn Plains, as that had quite a few levels stuffed into one world).
A Hero's Tail was a major improvement, but still suffered from being a recycled game. In fact, it's very fun, but not at all difficult. It took...2 days (and that's because I get headaches for playing console games too long) to finish. The levels were interesting, so were the minigames (I loved the ball rolling) and the extra characters were fun (though I'm not so sure about Blink), the graphics were cool, but it was just like...Spyro 1 all over again, except expanded. The final level (Stormy Beach onwards) was a bit too linear for my tastes, considering the rest of the game was using the old formula except without instant transportation via portals (which was a bit stupid, but a forgivable mistake).
I haven't played the Legend of Spyro games (and I don't think I will, at least in the short term) but I have heard that the last of the Legend trilogy (Dawn of the Dragon, I believe) is the only one worth playing. That said, I really don't have any privilege in ranting the game, but I'm just not a fan of a 'different' Spyro.
Oh dear, there goes my rant. I'll end it right here... o_o