Advance GenerationThe hallmark of sprite-based Pokémon gaming, Ruby, Sapphire, and Emerald set you off on an adventure in the watery Hoenn region with tons of diverse Pokémon to discover. Fans of Kanto can reexplore their favorite region in FireRed and LeafGreen. Here you'll find other fans who enjoy talking about the Pokémon, strategy, and techniques to master this generation.
Pre-E4 (after the E4, rechallenging them is the best way to grind in pretty much all of the games):
In FR/LG, I mostly use the Vs. Seeker on the bikers at the top of cycling road and the cluster of trainers down in the southeast corner - east of Fuchsia. And of course the two trainers outside the Spa on Kindle Road.
In RSE, I mostly just grind wherever I can find the highest level pokemon with the most convenient typings or EVs for whoever I'm working on, but I have a few spots that I tend to use more than others. At the earlier levels, I turn lots of circles in the little patch of grass west of Mauville on Route 117, then later in the long grass at the top of Route 120 - just east of Fortree. Then near the end, it's mostly in Victory Road. And I battle Ty and Gabby every chance I get.
Victory Road in LG and Gabby & Ty in Sapphire/Emerald. I loved tracking down Gabby & Ty just to stomp them in one turn and get that quick EXP. It was great until around level 40 but I didn't need much beyond that anyway because I just used them to evolve some stuff I had.
In FR/LG - On the way to Mt. Ember, there is a place near the Spa (quick healing spot) where there are five trainers together (one double, so actually four) and if you time your Vs Seeker right you'll battle them all. Also, the Cycling Road's entrance is the best without saying.
In R/S/E - Gabby and TY. Also Victory Road, since re-battling trainers then was difficult.
This goes without saying, the Elite Four Rematch is the best way to train in R/S/E, but the problem in FR/LG is there is no damn Elite Four rematch before we finish the post-game... darn...