I think Terrak is better than Keldeo. I think Keldeo should be A+ Rank. It's counters are just too common, rendering it dead weight too much of the time. It basically needs Pursuit support to pull off a sweep most of the time. It's STABs aren't great and it gets walled by Celebi, Jellicent, Tentacruel, Gyarados, Toxicroak and Dragonite (maybe more). It can use Hidden Powers to get past certain threats, but it will still be invariably walled by something. I find it far too easy to counter to warrant S Rank.