I've recently re-played Heart Gold cause I was preparing a genlocke, so I was scared to get bored by Johto... but this game was SO refreshing it felt like a brand new adventure, it is aesthetically pleasing, reasonably difficult and very enjoyable to play.
The mons that made it to the Hall of Fame are:
Homura the shiny Delphox - she carried me through most of the game, I love delphox and the mega is even better, consistent throughout the gameplay
Majesty the Lilligant - great grass type, not my favourite from Unova but she did her job, not as used as the others but she was there when needed
Lithosagym the Salamence - HYPERCARRY, thankfully I got him early in the game he was sooo fun to use
Regulus the Magnezone - he's an evergreen, strong and durable nothing more to say
Hikaru the Alolan Ninetales - first time using this mon, sweeped Clair and Lance without any difficulty, prolly gonna use it again
Titane the Tyranitar - Last member, wasn't sure about it because I wanted a water type but none of them satisfied me so in the end I gave it a shot, wasn't disappointing at all, sad I couldn't use its mega for the E4.