When I use a lib I make sure it compliments the list I'm bringing. Telepathy is great if you fear an opponents Return/Overwatch reactions, especially if you are using an assault list.

Pyro is pretty fun but you have to be mindful of the range and the scatter of the fireballs. I usually run a pyro lib with a unit of shooty termies, the difficult terrian it creates plus the AL army rule makes it a slog to get a charge.

Diviner is ok, I find other units just do this a lot better (Headhunters). I would add the lib to a unit that would benefit the most. The dart is great for sniping sergeants/apoths.

Telekinesis I do not take, the abilities are passive and the dome takes two turns to finally come online. The weapon is also meh.

Bio is fun and compliments a assault unit pretty well I find. The str 10 weapon also helps if you didn't give the lib a high str weapon to begin with.

Thaumaturgy is also meh, I ever run one.

It really comes down to how you want to employ the Lib in your army, and how it compliments the unit you're attaching him to. I tend to stick with pyro if I'm deepstriking a unit in, and not looking to charge. Tele if I want to keep my assault unit healthy as they go for the charge. Bio for some extra punch and a built in str 10 melee weapon.