As far as realms go: Pure F2P (Starter Edition) players want to look to Aerie Peak, Bleeding Hollow, and Dalaran on the Alliance side for connecting with other Starter Edition players. Horde side the realm choices are: Bleeding Hollow, and Dalaran. These realm options are also solid choices for paid players, because there are (or were at one time) a good amount of guilds focused around the 20-29 Bracket.
Other realm options for paid players with Guild Consideration:
Alliance:
Darkspear (There was a guild on Darkspear that was active during Legion: <ftp bullies>)
Horde:
Illidan (There was a guild on Illidan that was also active during Legion: <Citadel of Twinks>)
However both of the "Alternative" options listed above (Darkspear and Illidan) might be a shot in the dark, probably better off rolling on Bleeding Hollow or Dalaran (+Aerie Peak if going Alliance). I have also seen some recent activity on Area 52 (for Alliance side) and Zul'jin (for Horde side), just not sure how big or sustainable those realms' communities are.