1) VGA Box 

• Pros >> The Dreamcast actually boots up in VGA mode, a feature which is programmed into the game.  The VGA mode offers the highest possible resolution for Dreamcast games.  This is easily the best picture because you can see the 640x480 resolution clearly, without interference, on a VGA monitor.
• Cons >> Since a VGA mode has to be programmed into the game, not all games are compatible with a VGA Box.  Usually monitors are smaller than TVs and you have to buy this device separately.

2) S-Video 

• Pros >> Offers the best possible TV connection which is currently available.  This is because S-Video separates the sound, colors and luminance signals.  This is perfect for owners of high-end TVs.  NOTE: Since the AV out port on the Dreamcast is programmable, future video connections may become available, which can take full advantage of higher resolution TVs (like Digital TVs and HDTVs).
• Cons >> You need a TV with an S-Video input (usually the most expensive).  You have to buy this cord separately.

3) RCA (composite)

• Pros >> Offers the second best TV connection which is currently available.  This is because RCA separates the sound and video signals.  This connection is included with your hardware purchase and is perfect for owners of normal TVs.
• Cons >> You need a TV with an RCA input (slightly more expensive).

4) RF (cable)

• Pros >> Offers the minimum TV connection which is currently available.  This can be hooked up to any TV you're likely to own.  
• Cons >> The sound and video signals are mixed, which usually causes loss of quality and imperfections in the video signal.  You have to buy this cord separately.