Not the end all be all answer, but I used a 20" LCD monitor in my first cabinet. I got a 27" TV with component inputs for my second larger cab. Before I could get it completed I found a Wells Gardner D9200 27" multi sync arcade monitor.
By far the best picture is on the arcade monitor. Being multisync it is very easy to use, has a common 15 pin computer VGA input on it so it hooks up just like a computer monitor. I currently only use it at the 640 x 480 resolution. I am thinking about getting a ArcadeVGA card so that I can run native arcade resolutions.
I would recommend the largest multi sync arcade monitor you can fit in your cabinet. I found mine on Craigslist. I found a person selling a beat up old cabinet with non working video for super cheap and took a chance. The monitor looked like it was in decent shape. I got the monitor out and it needed a new cable which I had to build, but it works great now and it really makes the other options look inferior. I mounted it horizontally and with that size it plays horizontal and vertical games well. No need to rotate it as vertical games are almost all 19" and this ends up being about 18 3/4".
They are getting harder to find, but I think you may want to look at
www.niemandisplays.com They have a 24.8" that would probably fit your cabinet.