Xen
Wi-Fi Leader
Fire or ice for the cute version...not sure about the weird uber dog thing
Thanks for the responses. I think I'll do HP Ice since Shaymin does get Earth Power through G5 Tutors, which somewhat covers HP Fire anyway.HP Fire allows Shaymin to hit Ice-types but HP Ice is great as well as it hits Dragons.
EDIT: Actually, I'd go with HP Ice as STAB Seed Flare hits harder than BP 60 Super-Effective HP Fire.
Also, with the Dark type attack buff and Darkrai gets Focus Blast it doesn't really need any hidden power.
Darkrai also learns Incinerate (can't believe that's actually better than HP Fire now. ;_;)