Which version of guest-editions and extension packs shall I installs

Hi all,

Host - Ubuntu 16.04 gnome desktop
VM - Ubuntu 18.04 gnome desktop
VirtualBox Graphical User Interface Version 5.1.34_Ubuntu r1 (VM manager)

Please advise which version of guest-editions on the repo shall I install ?
and
which version of extension packs on the repo shall I install ?

Thanks

Regards
satimis