The problem is Hiruzen has never actually shown a certain type of KKG which is extremely powerful or has never done something "great/epic" for example:
Hashirama: Ended wars, fought Madara and the Kyuubi, founded Konoha, could handle Bijuu's like pets (he had like what? 7 Bijuu's in his control or something like that?) Superhuman strength (I think he is stronger then Tsunade or atleast equal in terms of strength)
Mokuton Jutsus (the clones only Madara could see through with the Eternal Mangekyou Sharingan) the Scroll of Seals which contained alot of Kinjutsus, leader of the Senju clan along with the Uchiha's they were the most feared clans.
Tobirama: Master in Suiton Jutsus, did Water Dragon Bullet Jutsu in 1 handseal, creator of Edo Tensei (some time later after he created it he labelled it as a Kinjutsu so he didn't master it himself, but he did use it because Muu knows about it)
By just touching the ground he could detect how many enemies there were around.
He had S/T Ninjutsu
Hiruzen: Enma... He fought both Hokages but he didn't last too long, I mean Hashirama couldn't even use his Mokuton properly because of the barrier, he barely did any Jutsus and Tobirama didn't even use S/T or perhaps he didn't need to?
Minato: When people saw him in the war they immediatly had to retreat, fastest Shinobi in the Narutoverse, creator of the Rasengan and FTG technique, Sealing Jutsus like Contract Seal, Dead Demon Consuming Seal, Eight Trigrams Sealing Style, Space Time Barrier...
Defeated Tobi, making him retreat and foiling his plans, sealed Kyuubi and so on...
Tsunade: Well lately she's been doing great but still Jiraiya deserved the Hokage title much more however he didn't wanted to be Hokage or something like that.