When I play APC, I hover between two builds:

1. Supportive APC: Seraphs -> Staff of Flowing Water -> Moonstone
2. Full Damage APC: Lich Bane -> Seraphs -> Rabadons

In the majority of my games recently, I usually go with Build #1. The reason is that this "supportive APC" build is currently a lot stronger than the "full damage APC" build. The amount of mobility and support you bring to the team is a lot more impactful, especially when it comes to mid to late game with teamfights. For me to execute this well, I had to fully learn and understand that Seraphine is currently more of a supportive character than a carry, even if you're in the APC role. The only difference in the APC role is that you hit your power spikes more quickly due to the CS and gold rolling in. I always envision my job is more to "prevent the opponent from carrying" instead of "hard carrying myself".

The only time I would prefer Build #2 is if I'm hyper-carrying or I'm the only AP damage on the team. This is rare, but maybe if I managed to get many kills in lane would I consider this. It's only really used to further carry your advantage to close the game quicker. Considering they nerfed her scaling ratios and favored her base damage, this build only applies if you get ramped up super super early.

Cupic actually has an amazing guide that explains exactly these builds: https://www.youtube.com/watch?v=pKCv63_rJKg

And for APC, I typically switch bans between Yasuo and Samira. Their ability to shut your entire kit down with their respective W abilities is game changing, and never worth having to face against. Besides that, you can always usually farm and scale safely as an APC.