If you don't have a cab already look for ones that are convertible from open to closed.
Different rooms, placement of the cab in the room, and open or closed back can all change the tone.
With options, you can see what you like better.
A few months ago I did try out quite a few speakers with a Marshall 1974x. To my ears a 68 pre-rola 75hz G12M suited it best. I found the G12H speakers to sound a bit dark with that amp. The smaller magnet seemed to suit it better. In close second place was a 1975 G12S 75hz speaker which also performed very well with it.
Scumback M75-PVC 65W. Had one paired with a similar amp (Avatar 18W) and open-back 1x12" cab years ago and loved it. Wish I could remember the other speakers I tried. I just recall that one being far and away my favorite.
If you are buying a used/vintage speaker, I'd go with the a Holy Grail G12H30 Greenback T1281 but they are pricey.
But really it depends on the cab, open back vs closed, large vs small, ported etc...T1281's usually work in most cabs.