Shanks was never the best of his era or generation
That's why he's a failed Roger
The strongest of Joyboy era : Joyboy
The strongest of Golden Era : Roger
The strongest of Great Pirate Era : Newgate till he became sick, then it became Kaido
This era : kaido for 2 years ...now there's no actual top 1
Now about generations :
Void gen : Joyboy
400 years ago : Noland / Ryuma
Old gen : Roger
Midgen : Kaido
Newgen (worstgen) : will be Luffy but currently it's Teach