Several years ago I would have said an old
Micomsoft linedoubler
from best to lesser for arcade boards:
XC15 DISPL (that one might require some RGB input modifications)
XRGB-1 (revision with 3pin/white switches on the back)
XRGB-2 (okay but might cut out a few lines of the resolution)
XPC-4 (nice but a bit complicated; read about it over at
http://retrogaming.hazard-city.de/)
Problem; they're all rare and overpriced...
Today the go-to linedoubling machine is the
OSSC, it far outclasses anything that existed before, outputs HDMI and produces its own simulated scanlines and CRT effect as per the latest firware. You can also use it over DVI or even VGA with a simple adapter dongle:
http://junkerhq.net/xrgb/index.php?title=Analogue_video_output_options_for_OSSCFor input unless you already have something that outputs it (like a supergun with scart-rgb out or jp-21) you might need a kind of converter to SCART.
There's an official store and forums called videogameperfection, but recently some slightly cheaper chinese knockoffs have appeared on websites like alibaba (quality/reliability unknown)
The thing with linedoublers is that they double (or triple etc) the resolution but carry over the video signal's same refresh rate, and the Neo/MVS outputs 59-something which a lot of displays like TVs don't like and might refuse to show.
Though PC monitors are usually more indulgent, in particular at lower multiples (linedoubling, linetripling) and might accept to display even sharper multiples (x4 and x5) so your chances are still good.
There's hardly anything else to choose from if you're seeking something better than what you currently use (the GBS-like converter pcb's), even slightly so means digging for hard-to-find obsolete scalers that will offer barely better results and about as laggy as the GBS boards. And really most everything you'll find on amazon is no better than the GBS no matter the price.
Or you could go with the following GBS mod project if you're into tinkering with Arduino's:
http://shmups.system11.org/viewtopic.php?f=6&t=52172 (quite the hassle for litte gain IMHO)
Well, good luck. ^^