Welcome back!
Relative Realm populations are well shown on
(sort by Horde)
and
Draenor would be my choice, (and is in fact my choice for a Horde realm) but you might find a guild on one of the others.
Saneko is correct that the best realms can have queue times at peak login and around patches, but I haven’t seen any since the start of BfA, so I no longer consier that an important factor.
But what might help even more is for you to find a Community or Communities. Communities are like guilds, but they cross realms and factions (though guilds are going that way soon anyhow) and they are not exclusive - you can BE A MEMBER OF AS MANY AS YOU LIKE.
In Communities, you will find people to do different things with. I can recommend Scared of Dungeons specifically (Alliance) Scared of Dungeons EU EN but you may find others. Since you can join as many as you like, it costs nothing to try.