All-Time Favorites...
Singleplayer Levels:
1. Metal Flower
2. Mystic Realm (Nitric Citadel)
3. Acid Missile (Storm Ruins)
4. Blue Heaven (urm... no particulars)
Honorable Mention: Morph's Level Pack, Moon Temple, and Egg Palace...
Match Levels:
1. Multiplayer Fusion (Riverbed)
2. Space Fight
3. Carved Barracks
4. Feudal Age Rush
5. Jazz Attack (just not Lava Lake, Goo Lagoon, or Disco)
Honorable Mention: Industrial Sync, Jade Valley, Noxious Factory
Capture the Flag Levels:
1. Mystic's Castle Level
2. Sky Shrine
Honorable Mention: Railsniper Range, Concrete Stronghold
Circuit Levels:
1. Sonic R Remake
2. SRB2 Riders
3. Space Race
Honorable Mention: Noxious Woods
Please note that I am not counting miscellaneous contest levels that have been gathering dust. I don't remember any being extraordinary anyways. I'm also not counting my levels which were never released due to unfinished (well I consider it such atm) design... otherwise CTF might be fuller. Lastly I didn't count any CURRENT official levels. I have fond memories with JVZ and NFZ though... (and 'meh' memories with ZRZ)
Also note that the majority of these were not played in netgames, since I only ever play online in Jazz's server or once in FF's... I would play in more but I don't feel like downloading 8+ different Sonic's or dealing with a load of rubbish from some players. (there a way to ignore global chat?)
Now for detail... (1 in 10 viewers will actually read these)
SP: Metalflower has always been my favorite, and being discovered when factory levels were my favorite type, it has remained to be so. Acid Missile is just above Blue Heaven because the unique levels give it an edge... (water+springs, submarine+movers, basic+hordes)
As for match, again, Someguy tops it off with Multiplayer Fusion, including Riverbed which is great for somewhat larger battles. Space Fight was an old favorite, but I think that was before 1.09.4... Carved Barracks is the only level I REALLY liked in Spazzattack... with Feudal Age Rush being fun in OpenGL...
Capture the flag, I remember hearing some good things about a level by Mystic in a contest a while back so I dug it up and it looked really fun, but I never got to play a real game on it. I've only played real CTF's on the recent contest levels (2v2, 2v1, 1v1) and in Jazzattack (4v4, 3v4).
Poor circuit... I unfortunately don't see many good ones at ALL so my favorites are largely contained in race mods... (Sonic R, SRB2 Riders *coughwithexecough*)