To me, the cries before the 3rd gen had a unique (somewhat schizophrenic) touch that seems to be lacking in the majority of the 4th gen cries (barring certain legendaries). For one, who would've thought of giving Parasect such a distinctive (although reused) cry? Or Magneton, whose cry is actually somewhat like a melody? Lugia's cry sounds untouchable (if the developers ever think of 'remixing' Pokemon cries to suit the current sound capabilities of the handhelds). It's spot-on, despite the sound quality.
The 3rd gen brought a 'much-needed' improvement to sound, but most Pokemon cries seem to be merely grunts, chirps, low rumbles etc. They're definitely not as inspired as the ones from the first and second gens. The ones I thought that were truly pleasant to hear (as well as taking advantage of GBA's sound capabilities) were Manectric's and Sharpedo's, among others. Whereas one can 'easily' distinguish cries between Pokemon in the first and second generations (unless they were reused), it takes a much harder effort to distinguish cries of Hoenn Pokemon.
Thankfully, the DS, along with the fourth generation, brought an abundance of Pokemon cries and sound effects, thanks to the DS's far superior audio. Even the more natural sounding cries, like Turtwig and...that's about it, sound cute and fun to listen to now and then when fiddling around with the Pokedex. What's awesome though is the sheer amount of awesome...and somewhat provocative...sound effects incorporated into the Pokemon's cries. Tangrowth's sounds juicy and slimy, Porygon-Z's warped and otherworldly, Lopunny's...arousing, and the list goes on. The most impressive ones, as always, are the legendary cries. Regigigas' cry has an electronica touch to it, Darkrai's a very deep (you can tell when you slow down its cry) shifting cry and Heatran's akin to an edgy steaming kettle.