Oh, boy! Another one of these threads!
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Anyway… You have 111 Servants, 15 of which are SSRs. Beyond that, you basically have all roles we generally look at for these questions covered already.
Many people (such as @The_Cheeseman) would call the free SR ticket a “Nero NP upgrade” ticket, because Nero is definitely a strong gameplay and “waifu” pick.
I’m personally not that big of a Nero fan, but I must admit she’s usually a good pick. However, you already have one of her biggest competitions in the form of a grailed Heracles. I’m assuming Heracles is Bond 10 and likely 9/9/9 skills if not 10/10/10. If so, as an anchor, Heracles definitely has Nero beat. But, there are other considerations.
Personally, from a character viewpoint, I would suggest Salter, just because she’s awesome, kicks ass in Shinjuku, and has a great costume that we’ll be getting in a little over a year.
And she’s an alter. (I’m kind of a sucker for Alters. )
…Hm… That makes me wonder what Nero Alter would be like…