DVD's are 480p. The numbers are short hand for the number of pixels on the screen. The higher the number, normally the better clarity of picture.
Or 480i, which is two 720x240 fields interlaced temporally to make 1 frame. Interlace lines can sometimes be seen in fast-panning camera movements, sometimes scene fading, and other scene changes. Deinterlacing functions in TV sets or software can make 2 interlaced fields look like a unified progressive (480P) frame but the scan lines can more readily be seen.
There's also source quality material - a DVD or blu-ray at native resolution can still suck if the trv or film master used is of poor quality or, especially if put on blu-ray, a videotape source (480i).
TOS, TNG and Enterprise are all HD now. Best way to watch DS9? I think DVD or Streaming are roughly equal.
I'd go for DVD, with player and TV set both capable of doing good upscaling - some low-cost brands do terrible upscaling. In no order: Samsung and Sony often have the best picture processing functionality to upscale and improve video quality from a low-res source to a higher-res output. Still, gimme PowerDVD any day.
Streaming, along with higher compression and more compression-induced artifacting, has frames ripped out to improve bitrate usage. It really shows on videotaped material (30FPS down to 24 or 20). Film is 24fps but reduced frames aren't as noticed unless they rip out too many. 120HZ televisions try to artificially create frames by looking at two adjacent frames and drawing what it believes to be a midpoint, to make a smoother image but, whew, it's not perfect. It's not unlike that VidFIRE process the Doctor Who restoration team people used only a bit different...
