Which guaranteed banner should I roll for? (GSSR)

So, I'm not an expert or anything, so take my advice with a grain of salt.

Currently, your weakest areas are Assassin and Saber. The reason I say this over your lancers is because Lancer wise you have one completely serviceable ST Lancer in Shuwen and a AoE in Santa Lily. If you rolled Saber gacha, you are guaranteed a AoE Saber since all three of them are AoE. Assassin is fairly straight forward, you either get Jack who is best star gen or you get Jeanne who can tank all the things. You could roll Lancer also, but I think the Saber or Assassin one would be slightly better. Again though, I'm not the best at this so take other advice into consideration as well.

Your Assassin class feels so bare. Maybe consider an option there? Jeanne isn't so bad, she can tank most classes and has been my saving grace on numerous occassions. Whatever you choose good luck.

Also, I keep reading sob stories about lancer GSSR rolls getting Enkidu. I actually have him (on 3 devices) and he is pretty good.

Your choice is easy, roll Casters