Best B slot for Sonya?

Personally, I would lean towards Desperation. +Spd means she will have much better chances of doubling units if you're running Darting Blow, Fury or Life and Death. Vantage is situational at best and is rendered useless if she's attacked by a melee, whereas G Tomebreaker only really helps her match-ups against Summer Elise and Nino.

-L

It depends a little bit on how you plan to use her and what A Skill you want to run on her.

If for an arena team, I'd say Fury + G-Tomebreaker. Contrary to what someone else indicated, the HP test for Tomebreaker procs on initiation, not after the damage from the first attack. If the calc I ran is correct, Fury 3 + Tomebreaker is in fact the only way to survive a fully-buffed +Spd Nino (you are SoL if she has Moonbow up or is +atk since she kills on initiation in those cases). Also, one Fury recoil will not take you out of Tomebreaker range if your Sonya has to pull double duty and bait Rein first.

Running G-Tomebreaker will also cancel the effect when you run into Julias who run it almost exclusively in their B-Slot, allowing your Sonya to double her. Without it, you will be doubled. Julia will probably be everywhere after Hero Fest.

For other content (PvE stuff), Fury or L&D with Desperation or Fury with Vantage will work well. I have a -spd Sonya who will never see the arena unless she is a bonus unit again (I have better Julia, Nino and Cecilia) who has even been effective in the current Tempest Trials running Mirror Strike and QR2 (only running the latter because she is pretty slow at -Spd and was not generally doubling other mages in the bait in the TT).