I've played every Halo (*sigh* even wars) to its completion on Legendary (ok, so not wars) so my two cents:
- Campaing: Reach's campaing is entertaining but there's something missing. Maybe I prefered Halo 3's vehicle sections as they don't get that much action. In every other way, the campaing is incredibly entertaining. The final mission isn't very exciting though, unlike EVERY OTHER HALO (*sigh* including Wars). Halo 2 on the other hand, on legendary, seems like a chore many times, due to the inmense bull-blam!-ness of certain parts (sniper alley? WTF). I loved myself the vehicle sections though.
-Multiplayer: So I LAN a lot, and even though I had great times on Halo 2, I can do the same on Reach AND more. No brainer there. PS: Also, I get insulted less on Reach.
So yeah, Reach wins on both accounts. Too bad the campaing is so criminally short, but at least it's very replayable, and the challenges just add to the fun.