Best Vintage Speakers?

Infinity Monitor 2A.
Great bass response from a densely stuffed tuned cabinet and very nice treble extension thanks to the walsh super-tweeter, a la Ohm Acoustics.
Reasonable size and decent furniture-style look complements a great package. They're going around $400 a pair, but shipping can be expensive and parts are not easily available.
These speakers are very sensitive. I'm driving them with a small 3 WPC amplifier in a small living room. No issue with dynamics or bass response!
A great sonic jewel!

It all depends on size requirement of the speakers that can fit your room and the amps you have to drive them.

Vintage high efficiency speakers from the late 50s and early 60s are extremely desirable.
12" ~ 15" Tannoy, Jensen, Lorenz, Altec, Western Electric, Coxials
Vintage JBL, Altec, Jensen, Klipsch horns
Rogers and KEF LS3/5A speakers

If you like midrange and imaging, and can forego high volumes and driving bass then it's hard to beat Spica Angelus. You can find them used for under $500 and I would wager that for chamber music they would be impossible to beat.

They are quite tricky to mate with a subwoofer (I tried) but they have very controlled bass so they can be used in very small rooms (if the aesthetics is not a problem).

A great bargain.