First choice would be a multisync monitor that is 25" or bigger- great for playing the widest range of games (both horizontal and vertical oriented on the same machine) and feel like you are not missing something on screen size.
For a real arcade monitor... WG, Kortek, Makvision are the ones I was choosing from for my projects.
I think the Makvision is the only one available new anywhere.
My first few years in this hobby I had to accept the fact that all the vintage machines in Hawaii were scrapped long ago and finding an old monitor was like stumbling onto a unicorn.
But as a result of that, (and obviously not everyone's first choice) I have also had reasonable results with a regular old consumer CRT, the right video card/ CRT emudriver and an Extron scan converter.
Bobby