I think this would be a fun challenge The main characters get the bulk of the battles anyways, so looking at some of the more underrated battles.

For me, I'd probably have (not in any particular order)

Burst
Shu vs Orochi
Ken vs Ukyo
Wakiya vs Hoji
Kamen Blader Zac vs everyone (not counting Valt)
Rantaro vs Wakiya
Wakiya vs Xhaka
Gou vs Daigo
Shu vs Wakiya
Zac vs Akira
Lui vs Shu

God/Evolution
Rantaro vs Wakiya Rematch
Sisco vs Ghasem
Daina vs Joshua
Sisco vs Joshua
Shu vs Kurtz
Sisco vs Xhaka Rematch
Sisco vs Joshua Rematch
Free vs Lui
Shu vs Norman
Cuza vs Kurt
Shu vs Lui

Chozetsu/Turbo
Suoh vs Toko
Fubuki vs Souh
Xhan and Kyle vs Ranjiro and Suoh 
Laban vs Xhan vs Fubuki
Fubuki vs Suoh Rematch
Ranjiro vs Xhan 
Laban vs Phi
Xhan vs Phi
Free vs Kyle
Hearts vs Phi
Phi vs Free
Phi vs Shu

Gachi/Rise so far
Amane vs Fumiya
Joe vs Amane
Fumiya vs Lodin
Fumiya vs Delta
Delta vs Pot
Gwyn vs Joe
Delta vs Arthur 1
Delta vs Arthur 2
Amane vs Arthur (up until Big Bang Slash)
Gwyn vs Arthur
Amane vs Delta
Delta vs Blind

How was God the easiest to make out of all of these? This is actually a challenge since I genuinely enjoy these battles (if you have any questions about which battles I picked personally, I'd be glad to answer), but my tastes are different, so how about a Top 5 to make it easier.

Season 1: Lui vs Shu
Season 2: Lui vs Shu (Part 2)
Season 3: Phi vs Free/Shu (They’re pretty much the same thing)
Season 4: Delta vs Arthur

1.
Xander vs Quon
Shu vs Lui
Ken vs Ben
Zac vs Akira
Shu vs Wakiya

2.
Rantaro vs Wakiya (rematch)
Free vs Silas
Joshua vs Silas
Lui vs Red Eye
Free vs Lui

3.
Fubuki vs Suoh (rematch)
Phi vs Hearts/Free/Shu
Xander vs Xavier

4.
Delta vs Blind (rematch)/Arthur/Fumiya/Pot
Lodin vs Fumiya

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Top 5
Honorable Mention 5: Rantaro vs Wakiya (Rematch)
Honorable Mention 4: Free vs Silas (All)
Honorable Mention 3: Silas vs Xander (Rematch)
Honorable Mention 2: Clio vs Cuza (Rematch)
Honorable Mention 1: Shu vs Orochi
Best Honorable Mention: Free vs Lui (All)
5: Phi vs Hearts
4: Ken vs Ben
3: Fubuki vs Suoh
2: Silas vs Joshua
1: Shu vs Lui (Requiem battle)

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you better watch this if you want nostalgia over these years. MEP Legendary: https://youtu.be/BxXAlIpZ-ns

Burst:
Shu Vs Lui
Ken vs Ben
Wakiya vs shu
Diago vs Yugo
Ken Vs Xander
Lui Vs Ken
Ken vs shu
Orochi vs Shu
Zac vs Akira
Zac Vs Gabe

God:
Wakiya vs Xander
Daigo vs Joshua
Rantaro vs Shadow Rantaro
Free vs Lui
Masked red eye vs Lui
Red eye vs Lui
Xander vs Ren Wu
Free vs Silas
Joshua vs Silas
Lui vs Silas

Cho Z:
Suoh Vs Hae Jin vs Aiga but aiga burst in the beginning
Phi vs free
Phi vs shu
Hyde vs Phi
Laban vs Phi
Toko vs Suoh (6 way battle)
Suoh vs Fubuki (1)
Suoh vs Fubuku (2)
Phi vs Xavier
Ranjiro vs Xavier

most likely Delta Vs Arthur (episode 39-40) that battle had me on the edge of my seat

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PFP: Baby picture of me and one of my irl friends / ANY PRONOUNS

Not really per season, but my favorite battles excluding the protagonists are those:

GT: 10. Blind vs. Delta (ep 18 - 19.)
GT: 9. Gwyn vs. Joe (ep 33)
GT: 8. Gwyn vs. Arthur (ep 41)
Cho Z: 7. Phi vs. Laban (ep 24)
God: 6. Sisco vs. Ghasem (ep 26)
GT: 5. Arthur vs. Delta (ep 39 - 40)
God: 4. Cuza vs. Kurtz (ep 48)
Cho Z: Free vs. Phi (ep 43)
Cho Z: Shu vs. Phi (ep 47)
God: Shu vs. Lui (ep 49)

(Per season)
•Burst(1)
1.)Lui vs Shu
2.)Shu vs wakiya
3.)Xhaka vs wakiya
4.)Zack vs gou
5.)Kensuke vs Lui
6.)Xhaka vs Quon
7.) Zack vs Akira
8.) Shu vs jin
9.) Gou vs Orochi
10.) Shu vs Orochi
•God(2)
1.) Lui vs Free(ep.47)
2.) Lui vs Red eye
3.) Free vs Joshua(god cup)
4.) Shu vs Boa
5.) Xhaka vs Sisco(ep.42)
6.)Xhaka vs Sisco(ep.30)
7.) Lui vs Sisco
8.) Cuza vs Krutz
9.) Krutz vs red eye
10.)Gashem vs Sisco
•Chouzetsu(3)
1.) Free vs Kyle
2.) Free vs Phi
3.) Shu vs Phi
4.) Xhan and kyle vs Ranjiro and suoh
5.) Suoh vs Fubuki (rematch)
6.) phi vs Hearts
7.)phi vs laban
8.)Suoh vs fubuki
9.) xhan vs phi
10.) xhan vs fubuki vs laban
•GT(4)
1.) Delta vs Blint(1)
2.) Amane vs Arthur(1)
3.) Fumiya vs lodin
4.) Amane vs Delta(ep.43)
5.) Amane vs Arthur(2)
6.) Delta vs arthur(ep.34)
7.) Delta vs arhur(ep.39-40)
8.)Gwynn vs Arthur
9.) Blint vs Delta(2)
10.)Joe Vs Gwynn