I wouldn't dare go for Tactics rating, and I couldn't succeed even if it killed me. I like my support bonuses, thank you very much. So I play pretty casually.
Linus and Geitz, because 16x/17x and 20/21 Arena Abuse. Also gives me a reason to delay using Dart until needing him for Geitz, so then I can AA him to promotion with the desert chapter Ocean Seal.
Kenneth, because I end up grinding Serra to level 20 just for Luce. Same for Priscilla and Excalibur. Plus, I'd rather have Aura then two Light Brands.
Harken, because stronger character development and the Brave Sword. And BTW, I imagine the guy who gives you the Silence Staff to be Harken for no real reason.