what will be the best old CLASSIC amp to rebuild.

If the amplifier is for the AR3 speakers you mentioned in an earlier post I suggest that you refurbish the speakers first, before chasing after amplifiers.

The speaker crossover parts should be evaluated and replaced as needed and the woofer surrounds will most likely require re-doping in order to properly re-seal the speakers and bring them up to spec.

This is a really great idea to restore the AR 3 but if i
have not yet consering this question is just because i
do'nt know who is qualify to made a great job on the AR 3
in my area... Some of the best AR specialist are in the USA.

Searching the right tech will be my priority to achieve
this goal. The power amp was redone by a tech and the right
chanel hum and the tweeter do'nt work anymore on my right
AR 3 speaker... Not a good experience.

Same if the AR 3 was my best score on thrift store as i
paid them only $10 for the pair now if they are broken
because of a tech i feel like i have not up grade my
system....

Accuphase E202 speaks for itself.
Fully restored unit can be purchased at $800 that will stay on top of most modern priced upto $2...3k.

QUAD 303

When i receive the CM LABS 912 rebuild it was a deception.
But after 350h he became a complete beast of power amp.

Bass and dynamic are now the part of the equation. I had
add Virtue Audio speaker cable and order Furutech binding
post and IEC receptacle gold plated. I also plan to change
the five fuse with AMR high-end fuse, so who tell that the
audio hobby is not exiting...