Joffrey. Cersei. Tommen. Tywin. NONE of those fools had as much power as they think they did despite sitting atop the iron throne at one point or another, and three out of the four of them are dead (and let’s be real – Cersei ain’t making it to the end of the series).
So who has the most power in Westeros? Varys. Possibly Littlefinger as a tie, but Varys is the one pulling the strings and making secret alliances and moves in order to advance a Targaryen atop the throne. Think about it – who married Daenerys to Khal Drogo? Who facilitated giving her dragon eggs as gifts? Who brought Tyrion over to her side?
Varys is much more cunning than people give him credit for, and that’s part of his strength – don’t underestimate the eunuch.