In my opinion, both GSC and HG/SS are good in their own right. Although, if you really want to see the Gen 4 cross-evolutions, like Tangrowth and Mamoswine, you should go with HG/SS. If not, you can play one of the originals. Personally, I'd recommend Pokemon Crystal (unless you want Mareep, Girafarig, Remoraid, Vulpix, or Mankey, since they're not available in Crystal).