Okay, so if you're going to argue about Darmanitan, there's a few things you need to consider:
1) Sun support does not really exist. Sure there's Mega Zard Y, but you're stacking SR weaks with no defensive synergy for Pokemon that basically fulfill the same role. Sun is for all intents and purpose useless to this discussion.
2) Darmanitan is really frail and provides no defensive synergy to the team. Many hard-hitting offensive Pokemon/wallbreakers have at minimum usable bulk and useful resistances to take advantage of. This is a huge hit to its viability -- overall use for a team.
3) Darmanitan is reliant on a recoil-inducing move to dish out significant damage, and is weak to Stealth Rock. These factors combined mean it will die very quickly. Compare this to something like Kyurem-B, which is not reliant on recoil-inducing moves and has access to Roost for reliable recovery, which makes up for its Stealth Rock weakness to at least some extent.
4) Darmanitan has an unfortunate Speed stat combined with mediocre bulk/typing that leaves it vulnerable to numerous threats on offensive teams. This makes it reliant on a Choice Scarf or rather mediocre against offensive teams. A Choice Scarf is not very hard to exploit, for example a locked-in Flare Blitz provides setup opportunities for Mega Charizard X and Mega Gyarados, and a locked-in Rock Slide or Superpower provides easy setups for a lot of things.
If you combine all these factors together, Darmanitan isn't a very appealing choice for an OU team. It hits HARD but that isn't enough in a metagame filled with titanic attackers that overall contribute more than it to the battle. That's a no from me.
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This last bit is not related to Darmanitan, but to the thread itself. I ought to make this clear: comparisons to something as unviable as Rampardos should not be happening. Any possible cases for viability
must be supported with replays from the upper PS ladder (1800+) and/or from tournaments supplemented with a detailed analysis of what makes the Pokemon viable AND a worthwhile choice for an OU team. The latter part is of particular importance; recently the standards for rejection have been tightened. While many Pokemon are technically 'usable' or 'viable', only a select amount are actually worth considering for a competitive OU team to be used in a serious match. Only those Pokemon get analyses.
Adding on to what Jukain said, you can make calculations all day, but they're not sufficient enough on their own to justify using a Pokemon, it's all in theory (particularly when you're assuming every perfect condition to occur). Replays are worth much more when it comes to understanding how a Pokemon fares in the metagame. I'm not saying to go out and collect a bunch of half-assed replays about Porygon-Z against incompetent opponents, either. I'm talking about higher-level competitive battles that actually contribute to the tier. Furthermore, I don't wish to be mean, but I'm not getting the feeling that most of you actually understand the metagame enough. Examples like Blissey and Assault Vest Snorlax come up way too often for me to take some of you seriously.
There is a huge difference between "I can find a niche set for this seldom-seen Pokemon" and "I would take this into a serious, competitive match with the intent to win." Please learn this difference and post accordingly.
Please think before you post any more suggestions on this thread. You know who you are.
And again, if you want to contribute, PLEASE spend your time contributing to the threads we already have going, NOT begging to reserve an analysis for a shitty Pokemon.