*4 Star Free Servant Dilemma

Just how people asks what *4 Star Servant they would like to choose from,I'll do the same ...Since I got some of them-

Here's my usual lineup:
Rider:Iskander/Martha Assasin:Carmilla Berserker:Heracles (Sucks,Right?)

As I read some same Q&A about this,So far some says that people should go for servants that they don't have and not np level up.Their recommendation are either Emiya(Since,They said he'll be stronger in the future) and Elizabeth (For buffs). Can you please help me,Thank you