Come-on Dinh, let's not over complicate this. It's a simple question, in-fact members can even elaborate on why they consider their choices as 'top hobbyists'.
Erwin, to answer your quest, I really want to know the concept of "top hobbyist". Some people says that who are koi kichi with have resources, and willing to pay 4 or 5 or more figures to have top quality koi are "top hobbyists", but other think the top hobbyists are koi kichi that have learned much about koi and have excellent husbandry skills, and able to pick good tosai to raise to high quality show koi etc.... So I will have different lists for different concepts...
Like Erwin said, "Keep it simple" So, if you feel a "top hobbyist" is one who knows the selection process and maintains the quality through growing skills then that's who you should mention. If you think they're the "high dollar" hobbyist who simply buy expensive finished fish for shows, then mention those folks. It's easy! Just don't ask me to do it!!LOL
I cannot name five--only one name comes to mind: Richard Rombold. He has an artist's eye, breeds to please himself before anything else, and produces what I consider world class koi.