I love all of them. People usually like the same few pokémon from gen 4, but there's so much more that are cool!I love Finneon and Lumineon! I knew I wanted one on my Pearl team before I got the game. There's drawings of them made by 10 year old me, along with the rest of the team I wanted - and it's been on the main team for some 11 years now. He's in charge of Surf, Waterfall and Defog and has his own ball capsule. I've seen people call Lumineon forgettable - how?! It's so pretty!
Burmy and relatives is another one I don't see mentioned often, and Burmy is straight up one of my favourites from gen 4! The cloak gimmick, the gender-based split evolutions, being found in honey trees... everything was so cool about them!
Snover! I don't see it praised much, but it's so loveable and its "eeyoo-eeyoo" cry takes me back to my childhood. I used Abomasnow in link battles with a friend and it always came through with the 100% accurate Blizzards.
Lastly, Lanturn! Lanturn's been on my main team in Ruby for like 15 years, starting out as my resident Diver and Waterfaller, and I love it to bits. I used one in the Isle of Armour online tournament and it worked very well for me. Lanturn's HP is huge and it has awesome wide coverage, all it needs is a bit of extra offensive oomph and it rocks.
A pokémon I always loved is Kabuto, but I never found someone as enthusiastic of it as me.