Must.
Firstly, I absolutely adore Beautifly. It's been a favorite of mine ever since I first saw it in the anime and I tried using it in-game on several occasions despite how much it paled in comparison to a lot of other Pokemon. Still, as someone who doesn't usually like bug-type Pokemon, Beautifly was gorgeous.
Then there's the Taillow line. Although it's very frail and easily knocked out Swellow does a great job at hitting fast and doing great damage; it was very reliable for me, probably my most-used regional bird to date. Taillow was immensely useful early on, too.. and has a great design to boot.
Skitty is amazing. What did you expect from me? [': It's very cute. I had a hard time catching one since it was pretty rare to encounter and never wanted to evolve it because I found its first form to be much cuter. But Delcatty is nice too - had one with a similar moveset to the one Delcatty that had an episode dedicated to it in the anime, with Thunderbolt, Solarbeam, and Blizzard.
Slightly biased towards steel types but holy moly Aggron is awesome. So much bulk and tons of resistances. It's really hard to train in RSE but worth it with how useful it is overall. I loved mine.
Awesome dragons here I come. There's no doubt Hoenn introduced some of the coolest dragon types ever, pretty sure I used all three of these (Flygon especially) 289686878 times. It wrecked anything and everything. :D
Zangoose is very cool-looking. Perfect for my cool-wannabe younger self lol, it always impressed me how much physical damage it did. Using a move that normally does next to do no damage like Quick Attack and watching it take out tons of HP was all the more amusing.
It's forgettable but so cute. Enough said.
Regice was one of my MVPs back during the R/S days. Its special defense stat is off the charts - special attackers did next to no damage to it and I had a lot of fun tanking that way. Trained mine to the lv. 80s somewhere.
And lastly, my favorite of the bunch, Banette. Back then ghost moves were still physical-based so Shadow Ball was absolutely insane with how high Banette's attack stat was. I loved him so much I trained two of them to lv. 100 just because I could.
There are others I like as well but this post is getting a little bit..! Would be here all day.