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I've played every generation all the way through, starting with R/B/Y and to me none has been more fulfilling in my opinion than Gen 3. There's a huge diversity in environments, and for whatever reason the Pokemon in R/S/E really impressed me. There is something about R/S/E's designs that are more sophisticated than its two predecessors, without seeming overdone like in Gen 4/5 It's like Gamefreak.
Although Pokemon games are sort of inherently simple, I distinctly remember the Gen 3 gym battles and Elite 4 being distinctly more challenging than other generations, for instance in R/S, the champion uses steel types, which can be quite a pain if you aren't prepared.
There really isn't a reason that you shouldn't get the game, as anon mentioned earlier in the thread the game not only won the popularity contest on /vg/ but has been praised by other sources as one of the best games in the franchise, and its soundtrack won awards, despite newfriends spamming trumpets.
The two rival teams Magma and Aqua catch a lot of hate because people overly dramatize their goals. But in actuality Pokemon eco terrorists is one of the most developed and cohesive plots in the franchise. Team Rocket pales in comparison and Galactic and Plasma are highly less believable, even in a fictional reality.
I hope I was able to help.