What banner to summon for?

As always disclaimer: Roll for servants you like for whatever reason be it waifu reasons or gameplay.

Now that that's out of the way, summer banner part 2 isn't a bad one to roll for. You can roll for part 1 as well but the servants in 2 are just better. If you really need a rider surfer Mordred is good in part 1 and Tamamo lancer if you can pull her is good too. But squirtle aka squirtoria aka the squirtinator (Altria Archer) is one of the best Archer units and Martha ruler is the only 4* ruler available in NA right now and is quite decent as well.
Regardless, focus on getting yourself the welfare servant and getting it to np 5. Assassin shishou I mean Scathach is a good addition for a team without an assassin.

The current banner has a decent 4* Archer and 2 basically strong at farming gorillas in Gawainn and Titoria.

Lastly, I would definitely try to have completed as much of the story as you can to widen the servant pool in case you miss on the servants in the banner.

You're pretty much lacking in every servant, so I suggest waiting for summer as it contains a lot more useful servants than the current Camelot 2 banner according to the people around and are much solid than what's in Camelot banner 2. Well if you like Altria's lancer as a servant then drop a ten roll if you want.
If you then plan for summer which appears in just a day for two you could just pick one of the two, but honestly I'd aim for the first banner which contains Tamamo lancer, a really good lancer, but is somewhat hard to use as they have more servants, but if you want some support and more assured chance due to lower number of servants aim for the Artoria Archer (could support)as it contains Marie too (decent tanky caster) and a nice ruler servant which could carry you pretty far.
Well summer is mostly for Waifu so I suggest looking up their voice lines and their appearance.
Good luck on your rolls.