Who Should I Roll For?(Banners up to March 2019)

You don't have an archer, and Gilgamesh and ishtar are both great archers, so there ya go. Also, Musashi is a pretty good ST saber, and Scathach is a good ST lancer. Finally, you seem to lack a good offensive rider. Iskandar isn't really that great, but he's better at dealing damage than everything else you've got. Personally, I'd say for gameplay:

Gilgamesh > Ishtar > Musashi > Scathach > Iskandar.

Outside of that go for who you like most.

If you meant the GSSR banner, go for assassin+extra, rider, berserker or caster I'd say, since there are some great units available on all three and you are lacking a bit in these departments, you really can't go wrong with any of those. While you could also go for archers, personally I think the units available on those other 4 banners are just a lot better than anything you can get on the archer banner.

If you want a priority for this, based on your lineup:

Caster > Berserker > Assassin+Extra = Rider > Archer > Lancer = Saber

Ishtar, Merlin and First Hassan are all good servants. They should all have had their banners by March 2019.
There are others, but these are the ones I'm personally saving up for.