Who should I take to 5/50 next?

niceguy22

Summoners,

I have recently had a lucky streak. I took Heimdall to Rank 5 and pulled several solid 4* champs. Of the champs listed below, who should I work on after I take Heimdall to 5/50? Worth mentioning that I am halfway done with 100% clearing act 4 and am working on building my solid AW roster.

Thanks!

XxLoganTDCxX

Doc Oct is worth R3
Mephisto R3
Black Panther is trash R2 at best