I'd suggest Staraptor over Togekiss for in-game playthrough, although you do end up with 3 physical and 2 special Pokemon. However if you plan on including Giratina then you'll have a special attacker as well, as you're probably not going to use Bibarel for the E4.
Staraptor is very fast and hits hard with strong STAB moves and doesn't require you to have any other prerequisites for evolution ~~(like making Togetic learn Ancient Power)~~ (like the high friendship and a shiny stone).
A moveset of Fly, Return, Close Combat, and Quick Attack would be great for use in game.
Overall I feel Staraptor would get more damage off on your opponents than a Togekiss.