Who would be the members of your Small Council if you could choose any character you wanted? You don't need to limit yourself to characters who are still alive if you don't want.

I'd go with:

Hand of the King: Tyrion Lannister - because GoT S2/B2
Grand Maester: Samwell Tarly - he's a good lad, I trust him
Master of coin: Petyr Baelish - no, I don't trust him, but he's good at his job and I have Tyrion to watch my back
Master of law: Mace Tyrell - smurf if I know, he just strikes me as a lawmaker for some reason. Never hurts to be in a Tyrell's favor
Master of ships: Yara Greyjoy - she wrecks stuff
Master of whisperers: Varys - he serves the realm
Lord Commander of the Kingsguard: Barristan Selmy - because he's the goddamn Ser Barristan!

Your turn.