Definitely Nero! She’s a fabulous all-rounder servant, being useful for farming, normal quests, and especially as an anchor servant. She can also solo the final boss of Camelot pretty easily, if necessary. She only gets better in the future, with a 30% NP battery getting added in a year or so, she she’s a fabulous investment.
Being a generalist, she probably won’t be top of the list for mature accounts, but she’s amazing for newer ones. Also, she doesn’t know that she can die, so she just doesn’t.
Too-moe is also a solid investment. She’s not super outstanding yet, but she is a very good buster-meme Archer, and will eventually receive an NP strengthening that makes her among the top burst damage servants of her class. Great for bullying Sabers.
Other than that, the standard “who should I raise to prepare for Camelot?” advice applies. There’s approximately 2-3 threads on the subject each month, so pick a couple of your favorites to read over and you will be ready to rock!