A real arcade monitor would look best, when running at the games native resolution. For this, you would need a video card that run at 15Khz and AdvanceMAME, or you would need to buy Ultimarc's ArcadeVGA card, which is easier to use than AdvanceMAME.
A TV Out can look OK, but not good compared to a real arcade monitor. When using a TV, use SCART RGB or Component Video, for best results. S-Video can be acceptable quality, but Composite, Coax, and RF inputs usaully have very poor video quality on TV Out.