Obsidius, Gray King, or TPK, emphasis on the first two.
Forget about Ra/Infernus. As far as I understand (through playing an alt past 1000), there are enough ways to get Souls for weekly events (particularly through Delves) without Ra, assuming you’re not desperate for Dawnbringer (you don’t need to be) or trying to Medal all of your troops. As far as the old thread mentioned above is concerned, it’s exactly that – an old thread – and while Pharos-Ra is excellent for earning Souls (he was my first craft!), I don’t believe him to be a necessity for the general purpose of playing the game.
TPK’s fun and can make some builds viable, but you can usually get by without him, whilst Obsidius and Gray King are strategically clutch.
Also, search the forums for some Vash’Dagon inspiration. While not always the quickest or most optimal build, you can make some reasonably viable/decent teams with him.
- What are your best teams with the latest troops/weapons/classes? - #5 by XLS78
- Any tips for the upcoming Vash'Dagon troop? - #25 by esslee
I suppose his synergy with HKI could make HKI an option for you, but
Need, of course, being the operative word – do what you want/live your best life 
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