I just had the good fortune to roll my first V leader, and now i'm wondering where you could best take advantage of that. As I understand it, you can basically place a V in any role and they will very rapidly become hyper-competent at it.
Perhaps a research roll? Model design, so all future models have amazing structural design? SHQ, to increase PP and BP?
As others have said, it depends on the leaders stat distribution. I've been evaluating positions by what has been a bottle neck for my empires. Every time, that was my combat or military research. With that said...
Assuming all stats are equal, I put mine on SHQ Commander. The bonuses you gain on every combat roll is overwhelming. A +300% on every attack is incredible. And it just keeps getting better the longer the game continues. Then you get the +xp artifact, and you're rolling cities with a few infantry.
A close second would be military research. Getting key attack + defense techs quickly and distributing them to your troops before your opponent, is incredibly powerful. Especially leaps in tech, such as laser weapons. In my current game, I've been waiting for Polymer Armor and Heavy Combat Armor to support my next timing attack. I could have done this much faster if I had a better military research leader.
After that, I'd considered applied science or interior. I love interior. Tax and tariffs all day, money not a problem. If it is, sell rare metal (thank you recycling centers). Suppress unrest great. Interior policy great for pop happiness, etc. For applied sci, conventional gun optimization.
I wouldn't consider economic council. In all of my games, I haven't found my civ bottle-necking because I was lacking economic tech. I'm usually far behind my econ tech research rate. Economic policies for private investment strategem is great. That's 2-3 buildings for a few pp. I'd put a cap 3 or 4 there with high science, high intelligence, and if possible inventor.
So many options.