I use my leader (Pikachu) most of the time, but I do switch to Blastoise every so often.
I usually take Absol, Duskull or Ledyba with me - or sometimes Linoone.
I always stay with Mew as my leader, and only switch him if I'm planning to feed him gummis in his friend area, and then I need to get another leader. Then I switch back