Keldeo; in my opinion, a cool Pokemon with much, much wasted potential. Yes a 129 Base Special Attack looks real good, especially paired with Base 108 Speed. His other Stats are decent, with 91/90/90 for defenses and bulk. Typewise, he has four weaknesses but 6 resistances. A fairly good tradeoff, especially if paired with a Tyranitar who is usually capable of dealing with 3 of those weaknesses by itself (especially Psychic). Movepool, as stated for Kingdra's entry in last generation's POTW, is the make-or-break point for most Pokemon, and Keldeo...I'm sorry, but you are the worst of the Muskedeer Pokemon, end of story. On his own (no TM or Move Tutor), Keldeo gets entirely Normal, Fighting, and Water type moves: not a problem in and of itself, but I have more fingers on one hand than he has learnable Special Attacks. As for boosting attacks, the only good ones he gets are Work Up and Calm Mind. Swords Dance, on something with a Base 72 Attack? Justified for it's Ability? I'm sorry, but Cobalion is better equipped than the one Pokemon guaranteed on most Brony's teams. Still, it can sweep or stall with what it's got. Hopefully the following movesets haven't already been suggested. (And no, I won't bother making up a pun for each set's name.)
Hidden Power (Fire)/Hidden Power (Rock)/Hidden Power (Dark)
Item: Choice Scarf/Choice Specs
Choice item sets; simple to make, easy to use. Secret Sword gets STAB, ruins Special Walls (with Super Effective damage on most, unlike Psyshock), good power and great Accuracy. Surf vs. Hydro Pump, pick based on which end of the reliable/powerful scale you're at. For example, I would personally chose Surf because it has MUCH better Accuracy and the PP has been tripled! (No, I couldn't resist that one.) Hidden Power...Okay, which Pokemon last generation had in it's entry something about having a crappy movepool if Hidden Power is mandatory? Anyways, it's here for coverage and filler. Fire is chosen due to the popularity of Ferrothorn and Scizor, Rock is an option if you can nail Volcarona before it can Quiver Dance, Dark for perfect coverage with Secret Sword. Otherwise pick your type based on what you need to hit. Icy Wind because Keldeo can't learn Ice Beam. The Speed reduction is a nice benefit, but yeah, Keldeo's movepool is THAT dire. EVs...do I have to say it? 4/252/252.
Hidden Power/Icy Wind/Substitute
Item: Life Orb/Leftovers/Wisdom Glasses
Calm Mind to boost your Special Attack (and Special Defense). Nasty Plot would have been preferable but one has to make due with what one has. Secret Sword and Surf have been described in enough detail by now. Hydro Pump is not mentioned because the horrible Accuracy and nonexistent amount of PP just screams, "come counter me Spiritomb I can't touch you at all!" Which, I need to say it, will make your boosts worthless if you do run into a half-decently defensive Spiritomb. The final moveslot, however, will determine if you are an offensive tank or an arguably better Raikou. Hidden Power and Icy Wind have already been talked about in the previous set, but Substitute blocks incoming Status' and allows you at least another turn to Calm Mind. After a few boosts, few Special Attacks should be doing significant damage to your Substitutes, while you sweep with boosted Secret Sword and Surfs. If used, run Scald in place of Surf so you can sorta tank hits from both ends of the spectrum (when the Burn kicks in, that is). Items need little explanation: Life Orb if pure offensive, Leftovers with the Substitute, Wisdom Glasses is the best compromise between bulk and power you can find. As for EVs...I'll leave that for someone who knows how they work better than I do. The 252/252 spread is good if you don't run Substitute but if you run it, well, Raikou's 124 HP / 116 SAtk / 52 SDef / 216 Spd spread is probably the best for it. If I'm wrong, go ahead and correct me.
As stated earlier, Tyranitar pairs up very well defensively with Keldeo. The dinosaur is often equipped with one or more attacks that KO what the colt is weak against while the colt hits most Fighting types at their typically weaker Special Defense. And as others before me have said, Keldeo works great on Rain teams.
† I am a Christian and proud of it! Copy and paste this if you are too.†
-Take up thy cross and follow Christ Jesus.
~Don't underestimate my God~
♥I believe in Jesus Christ as my Savior. If you do too and aren't scared to admit it, then copy and paste this in your signature.♥
I don't calculate stat values, I don't use cheating devices, I don't breed my way to perfection, and I don't care about natures. I catch my Pokemon the way they are, and treat them like individuals instead of brainless drones. If you use this philosophy, copy & paste this into your signature. (Started by Tyranitar, "I don't use "uber" Pokemon," edited out by me because I do use "uber" Pokemon on occasion.)