User blog comment:Akrie/Cookie Run Popularity Contest: Wrath of the Red Dragon Edition!/@comment-33658145-20160723041118/@comment-26044650-20160723042623

Because of the combi changes, a lot of relay Cookies aren't really good and/or situational as in the majority of the cases the combi bonus will only work with the main Cookie (due to the Cookie that can only do a specific thing that the relay cannot do) so it's up to you on what relay you want to use that you're comfortable with.

Fire Spirit Cookie seems to be a popular choice, but again, it's up to you.