Here are the best of Gen III’s Pokemon that are the ones you want to focus on powering up.
I will start by saying that Slacking (first form: Slakoth) is debatable as while it has huge CP its unique moveset means its primary attack doesn’t actually do anything.
• Gardevoir (first form: Ralts) is like a new Alakazam, but it’s not just psychic, it’s fairy/psychic, which means it isn’t weak to dark or bug which is what people bring to fight psychic. It’s only vulnerable to ghost, steel and poison which aren’t the types people have heavy hitters in.
• Aggron (first form: Aron) worth powering up as it’s like Golem but with a better type combination; it’s part steel, which is one of the best defensive types in the game. It also makes it a steel Pokemon that isn’t vulnerable to fire as it’s cancelled out by rock.
• Walrein (first form: Spheal) is like a Gen III Lapras. It can learn double ice to be good against dragon but it can also be double water. It’s good in two type categories that there isn’t a lot of competition in. Spheals are also everywhere and it rains a lot here, so it’s easy to get amazing weather boosted ones.
• Hariyama (first form: Makuhita) is like Machamp mark two but is more defensive and a lot easier to evolve and power up as it only requires 50 to evolve.
• Swampert (first form: Mudkip) is a pretty powerful water Pokemon and there aren’t a lot of those outside of legendaries so that’s worth it.
• Shiftry (first form: Seedot) might be worth powering up but only if you need a good dark Pokemon. It learns double dark moves to get the STAB bonus and it’s weighted towards attack.
All legendaries are of course worth powering up too.
TL;DR: Gardevoir, Aggron, Walrein, Hariyama and Swampert are definitely worth powering up. Shiftry maybe if you need a good dark Pokemon, while Slacking is debatable and I wouldn’t automatically do it just because it has high stats.
I hope this helps 