QSS if they have 2 or more champions with hard CC, FoN otherwise.

It depends on the enemy team. If there's a Veigar spamming Event Horizon, I'd much rather have a BVeil to walk right through the **** thing.

Really, you're only paying about 954 gold to block a spell every 30 seconds if you don't include the mana in the worth of stats. Depending on the enemy team, that may or may not be the difference in you doing your job effectively.

FoN. The Regen is amazing cause Shen has lots of HP naturally, and the MR is what you get it for (and there's a lot of it), but what I love this item more for is the extra movespeed. Helps me catch those runners for my allies and secure the kill.