I think it's a good idea to mix it up here: Study some endgame tactics, then some theory (these often blend into each other anyway), then some strategy, tactics again, etc., and repeat the material now and then.

If I had to choose one though, I agree with dfan and say endgame tactics. That's a seriously underrated chess skill in general, and many points are won and lost there in otherwise close games.

The mistake many people make is they automatically think endgame study = techincal endgames, study a bit of that from some large endgame bible, get bored with it and then stop. Most players would do well to memorize only the technical endgames up to their own rating level in Silman's endgame manual and otherwise concentrating on tactics and strategy instead, especially if they find that more fun anyway.

If you wanted to choose one area of study to improve your playing strength asap, which would it be - Endgame Strategy, Endgame tactics or technical endgames?