Improving team

Currently my team is composed by CharX, Giratina and Celebi. I also have Mega ampharos and I'm using my resources on him but I got lucky on some myst catchs and got a Dratini with -P.atk +Sp.Atk, another Dratini with -P.atk +Speed and an Yveltal with -Sp.Atk +Speed. I've seen many people use Dragonite for WB and monster riot and on my server there are many top ranking people that use Yveltal with a high speed. So I was wandering wich one should I invest my resources in, since I don't have enough to invest in all of them.