What format and frame size is your source video?
Theoretically, the best result ought to be to export in exactly the same size/format as the source material, so any parts where there are no edits don't need to be re-compressed. But on a fairly small monitor you may not notice much difference...
If the source material is standard-definition DV (e.g. from a MiniDV camcorder), then choosing "AVI" and the preset "Full screen (DV)" ought to give the best result. But file size will be quite large (~13GB per hour). For smaller file size, but still IMO good picture quality, try MPEG-2 with the preset "DVD Compatible". Other export formats have their niches (for compatibility with particular devices or to get smaller file sizes for uploading, but if you're just going to play the finished video on a PC, probably not worth using them as exporting may be slower...
Another thing that might affect video quality is the setting for "video and audio preferences" - if your source footage is DV, and Studio did not automatically set the background rendering codec as DV, set it manually (that setting also applies to stuff that needs rendering during the export, even if background rendering is OFF). I have observed occasions with DV source matertfial and the background rendering codec set as MPEG-2; there can be little glitches at the boundaries between original footage with no effects/transitions (so doesn't need any rendering) and stuff with effects/transitions/overlays (and is background-rendered, or rendered diring the export)
HTH
Richard