Whats the best vintage Yamaha amp to invest in?

There is no definitive "best" of anything. There are way too many variables involved. The "best" you can hope for is to find something that works well in your system. You can then say it's the "best" for you until you find something you think is better !!!!

the yamaha cr--xx20 series receiver is generally regarded as their classic--the subsequent cr-xx40 series are supposed to be sonically inferior and aren't as sought after. (their integrated amps of the same period go by the ca designation). i still own and enjoy a cr-1020, though like rlwainwright i prefer the hk and tandberg of the same era. from a collectibility standpoint, the most powerful yamaha units hold their value the best.

I agree witrh RW, the old Yamaha amps with the wood cases were very good. I had the CA-810. Yamaha was considered high end of sorts back then. There was a Yamaha power amo that had two huge meters in the front, that was a nice amp.

the old M series were class A rated and amazing. The one with the meters was the m-80. What a monster, it would power up to 3 pairs of almost anything. The 35, 60 & 70 are great too with the matching pre-amps. They compare to threshold in sound quality (not build quality). Otherwise, I'd go with the sanui 717 listed here.

I had one of them and sold it after six months. The sound was very thin and bright. If you want something from that era you should check out Luxman. The ones with the rosewood cases. I had the model r-3045 and it was night and day compared to Yamaha.