i don't think i've ever had a solo world. at least not one significant enough to remember. i think i got minecraft mostly for the servers, so my first memories are playing on mineplex (i was, and still am, a big captain sparklez fan) and occasionally the hive/hypixel. i was obsessed with survival games, even though it lagged like hell most of the time, and bridges. the first world i remember was on a multiplayer server owned by some boy i met on mineplex, named plum, and his brothers. he invited me to join, and later let me invite two of my internet friends. i still remember my first house, on the shore of the ocean. we built a stable, a doghouse, storage room, a glass pyramid library, a nether portal house, a mini-jungle tree-village, a tree-farm, sheep-farm, strip-mine, a giant rainbow (+pot of gold) out of wool that had a secret underground vault, a castle tower (like a rook from chess), a small castle for rabbits, a whole pirate ship, and an underground pathway running underneath everything. i have a lot of fond memories of that server, even though i've long since lost touch with those two friends, and we had to abandon it when plum took a weird turn and started being aggressive and griefing us. i went back to playing on mineplex et al after that.
my second world was my own server with those two friends, where we had some more fun and built a lot more stuff, but then i think it got deleted because i didn't pay for it or w/e. i have memories of a third world where i recreated a lot of the builds from my first/second ones, but i can't remember whether or not those friends joined it or if it was just my sister and i. my most recent world was with my girlfriend (and occasionally my sister), and i used a free service to run the server, and it was a lot of fun -- we used discord to talk and built a lot of cool stuff -- but one day we just... stopped playing. not sure why, we just did. i've no idea if the server is lost/deleted or not. if it is, i'm not particularly upset though. the best part of a world, for me, is always starting so.