(sorry for the lack of sprites, but no images allowed under 15 posts; an understandable rule to prevent trolling I guess!)
I have never been quite so randomly bemused as accidentally stumbling onto this thread. Except Sydian, none of you understand the slightest bit about the humor of the game that the sprites represent (absurd, expressive, free flowing at times), let alone the fact that the game was made to be about monsters. That word was included in the name of the game for a reason! There is a reason that sprites here are wicked looking, and also often laughing/amused. That this changed is not because the sprites became better, but rather because the games after GSC were solely, 100% guaranteed about preserving the series and the money flowing in, not about presenting anything fun or good looking. I am not saying every sprite in RB is good (probably around 80%?), nor in GSC (probably around 60-70%?), but it goes down to 10% or less in every game afterward.
Especially puzzling is people calling out Blastoise, Golbat, Haunter, and Kangaskhan in particular. Haunter is one of the few very best sprites ever made in any video game!
Blastoise is not pointlessly fat in the way that any monsters were in DP (Magmortar, Electivire, Rhyperior, Tangrowth, et cetera). The huge bulk is to make it an absolute bully, as you can see from its very well designed eyes. The mouth adds a bit of meanness, but also completes the absurdity and humor of it all. Even without reading into all that, one look tells you that this sprite looks good, unless you need a sprite to look "slick" or something.
Golbat is absolutely monstrous. The long teeth, the oversized tongue wagging out so ridiculously far, and the incredible length of its mouth...all completed by anime cat styled eyes, making the whole affair just a bit more absurd. The reason you are all so attracted to it is not because it is bad, but because you seem to think it needs to look "good" and slick again...
Haunter is the perfect combination of the humor and monstrous effects that the creators of this game were in love with. The wicked eyes, the intensely curled, clutching fingers, the sharpness of the whole creature, all with an obvious sense of Haunter having the laughing time of its life. The last part is lost by the next game (Yellow), and the sharpness goes nearly as quickly afterward. The effect is that one of the best sprites in video game history turns into the most generic rubbish it possibly could.
(Kangaskhan) "Oh my gosh, you're so cute, yes you are, yes youuuu are!" should be the reaction of anyone to these sprites. The eagerly excited mouth and eyes, the baby (especially when it is calling for mommy!), the cuddly largeness, and the awesomely clear detail of all the texture makes it so this sprite really stands out. After these games, that was lost, and Kangaskhan just looks like a clumsy bear like beast, kind of like an even uglier Ursaring.
Not every sprite is good in GRB, but there was a detail, a free flowing humor, and a sense of wickedness that was completely lost. In GSC, they substituted that with cuteness while retaining some of the humor. In ADV, it was completely sold out. In fact, a lot of monsters seemed to be purposely ugly in ways like coloring. Technology made it easy to use whatever colors they wanted, and it also made it easy to throw out sprite designs with absolutely no effort at all, so they just made everything possible generic and totally lacking expression. Apparently you all think this is a good trend, but technology is no substitution for creativity. HGSS is the absolute bottom, evidenced by sprites like Jolteon...we really cannot go much lower than this.