one county with max baronies
Y tho?
You can pop your steward on your capital county and get the tax benefits of hitting 5 castles instead of 1 or 2. Also the aforementioned doomstack you can get in your capital county, plus, again aforementioned, the ease with which you can move around your primary levies, given they're pretty much already together.
I generally do what ZG does: max out the castles in the capital county, then focus on the counties in the capital duchy. Also, setting the crown focus on your capital county when it's got holdings in all the available slots appears to give a chance of an event to pay a lot of pennies to open another slot, allowing you to build even more castles. After I have all the castles, I will then focus on retinues, regretfully, since I'm not a huge fan of retinues, since they just sit around doing nothing a lot of the time while eating your money/prestige.
5 castles gives you, like, 15k guys depending on your martial and tech anyways.