Underrated yes , not top 10 tho.His arsenal is not very big without Sage mode that takes way too long to activate for him ,i wish he was given this
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Depending on who you think the other nine are, I suppose.
But...
Base Jiraiya has:
Ninjutsu: 5
Taijutsu: 4.5
Genjutsu: 3
Intelligence: 4.5
Strength: 4.5
Speed: 4.5
Chakra: 5
Handseals: 4.5
In base he already has the highest known stats, tied with Itachi.
Then there are his jutsus:
Chakra Suppressing Seal
Dome Method Formation
Finger Engraving Seal
Fire Bomb
Fire Sealing Method
Five Element Seal
Flattened Silhouette Manipulation
Frog Conversion
Hair Needle Barrage
Needle Jizou
Oodama Rasengan
Rasengan
Rasenrengan
Shadow Clone
Toad Oil Blast
Underworld Swamp
Wild Lion's Mane
Summoning: Frogs/Toads
-Amphibian
-Dust Cloud
-Fire Blast
-Food Cart Destroyer
-Frog Call
-Frog Confrontation Singing
-Frog Hiding
-Frog Katas
-Reverse Summoning
-Sage Mode
-Sensing
-Summoning
-Toad Gourd Prison
-Toad Mouth Binding
-Toad Oil
-Toad Oil Fireball
-War of Words Binding
-War of Words Decapitation
-Water Bullet
-Wind Blast
He's already capable of fighting and beating any Kage-level ninjas, in Base Mode.
...Then he has Sage Mode, which amplifies his strength, speed, and endurance; gives him virtually limitless chakra, sensing, and also supercharges his ninjutsu, taijutsu, and genjutsu. He was strong enough to throw a boss-sized summon and fast enough to blitz a football field. Since his jutsus get supercharged, he can use jutsus like Chou Oodama Rasengan, Giant Fire Bomb, and Deep Fryer. The Ni Dai Sennin also act as batteries which gives him infinite Sage Mode, and also act like shoulder cannons and are genjutsu experts.
Also, Jiraiya took so long because he was trying to summon the Ni Dai Sennin directly into his body, we don't know how long it would take Jiraiya to activate Sage Mode without doing that.
However, Fukasaku said that he and Shima have a jutsu that allows them to fuse themselves to Jiraiya, so he could summon them and then just have them merge with his body.
In addition, I honestly don't see why Jiraiya couldn't have a Hoen Rasengan. You don't change your chakra into fire after you shoot a fireball, you do it before you shoot a fireball. So why couldn't Jiraiya change his chakra into fire and then use Rasengan? Doesn't make sense to me.