Creative - Spirited Away's Bathhouse, with the surrounding village (still building it though. server, y u no come back online?!?).
SSP - Lots of stuff, my friend thinks a castle I built (no editing/hacks) was the best. The lava death pit volcano-style in the middle of it is the most attractive in my opinion... good for sacrifices to the Nether God...
SMP - An entire underground city, in which you could travel by boat or foot along the passways, with lots of houses and shops, and almost invisible watchposts from which we could ambush the enemy if they actually managed to go through the vault-style entance (full legit obsidian). It had a palace included and took me 5 real life days to get the grass to go underground to the palace gardens. I built it all on my own for my faction, they loved it, but the server reset soon after... /sadface.