>>487977342 regions
Longer campaign
The first half is a linear story, while the latter half lets you roam the new region free as a bird-type (not noctowl)
Following pokemon
Pokeathlon
Leaf crown
Gym leader rematches
INSANE special events, like for rayquaza, celebi, arceus, etc.
One of the more difficult games
Moves Type/Category split, but pre-fairy
No shitty battle-gimmicks, no faggy affection bonuses
The rival is actually good
The best a pokemon game has ever looked TO THIS DAY
Full battle frontier from sinnoh instead of just some bland battle tower on its own
Pokéwalker
The world has been expanded in comparison to crystal
Speaking of crystal, it's essentially the crystal story & the gold story combined, no OR/AS bullshittery
Toggle-run
The pokegear radio
THE BEST implementation of the gane corner EVER, because Electrode-Flip is a super fun game compared to mindless slot machines
The only pokemon game that consistently has your team generally underleveled from your opponents, forcing you to think rather than brute-force curbstomp with levels
The only games to have your team at around level 90 by the end of the game
Trainer Red
The last good game before unovaborts ruined everything
I know you're just baiting, I know HeartGold is actually your favorite. but I enjoyed making this list anyways.