Oooo, favorite characters are easier.
Offhand, I'd say: Jasnah, Adolin, Marasi, Hoid, and Nightblood are all characters of whom I'm particularly fond.
Twinborn combinations...I'd have to think hard about that. Especially since some of the more esoteric Feruchemical uses have not been fully explained, yet.
I'm super tired at the moment, and don't sleep well in general, so Feruchemical Bronze is very tempting. Allomantic Pewter would be nice in terms of healing and generally feeling better. Those are fairly "boring" applications, but I'm feeling pretty beat from a late night last night and a bunch of yard work this morning, so that's where my head's at at the moment.
The Metallian