If you dislike addons and just want to one to target FCs, I'd definitely go with
Capping, as Kearbear said. Easy, unobtrusive, displays heath percentage and very little memory usage. Looks smart and entirely as if it's part of the Blizzard UI, so that's one addon that shouldn't bother you.
I use DBM for that, like Gattuso, but you might not want all the other utilities that one brings with it.
I actually use (too) many addons, and have no memory problems and don't find them too obtrusive, but that's personal preference, of course.
I wouldn't want to miss Battleground Targets for targeting/focussing the entire other team, but that's quite a bit more noticeable, so you might not like it. As Capping and Battleground Targets have already been suggested, I'll add SaySapped. Old, but still functional, and does nothing but make you say 'Sapped' when appropriate. I like it for FCing and teamwork. Cynics say that people who don't pay enough attention to notice their FC is sapped won't notice them talking, but I still like it.