"Best" depends on your monitor, the original game's res, your videocard, your OS, and other mame options used (like -effects or -artwork).
Current mame defaults to the desktop resolution, probably partially due to this is the best res for fixed res screens like LCDs, and the default for artwork, and if you want any effects.
If you have a multires CRT monitor, you can change the res, but mame doesn't do as well as it used to (and even that wasn't perfect, as there were variants that were better at picking res).
Enable -switchres and set the res with -resolution (and -resolution0, up to 3, for multiscreens). There are apps that help set every game's resolutions in there own ini files.