what will be the best old CLASSIC amp to rebuild.

I don't really understand what you are asking for here, other than you are looking for a good amp. You say that you never heard an amp you really liked, and that you even rebuilt a vintage amp and didn't like that one. Now you want to try and rebuild another vintage amp?

If the above is correct, I recommend you don't do anything until you actually listen to an amp you like. Doing anything else, just doesn't make sense.

""Best" and $1200 doesn't exist."

I think he meant the best you can do at $1200, not spend $1200 and get something that's better than everything else regardless of price.

"Do you think a rebuild classic power amp will offer better
money return than a modern power amp??? $1200 is just a
budget reference as i know some power amp are very expensive. Hope it more clear..."

I think it would be best to just get a current production amp. There's no way you can audition a classic amp before you have it rebuilt, so you will always be taking a chance. With a new amp, you can listen to it first. I think you can definitely make a better choice that way.

If you live in France, you probably know YBA. A few years ago, I heard an integrated amp from them and thought it sounded great. I remember that it wasn't too expensive. I'm guessing you can get one used for around $1200. I don't remember the model; maybe it was 201, but I forget. It was black and looked similar to a Wadia CD player.