As someone who used Donphan a lot last gen to varying effectiveness, I'll contribute my two cents.
Donphan was a bit of a dark horse in BW OU. One of the first lineups I remember to include it was Marth's famous SD Feraligatr team, which 3-4 players used to pop 1700 on ladder (myself included). The team had a 'phan running the typical 252/252 SR/Spin/Shard/EQ set, which performed surprisingly well against the plethora of HO teams on the ladder at the time. It was especially good against Sand Offense, which was probably the most popular playstyle at the time, being one of the only Pokemon that didn't resist but could still take 2 hits from a CB Terrakion CC. Meaning you could always switch in and play the 50/50 game with SR/EQ. That in itself was an incredibly valuable trait considering the severe lack of answers to CB Terrakion back in the day. Additionally, it had free switches on Tyranitar and Hippowdon, could reliably threaten out Exca back when that shit was legal, and even now does a fine job in the meta. It's an interesting niche fulfiller that operates off having things that other spinners don't, like the bulk and Electric immunity that Tenta lacks, the decent damage threat that it poses unlike Forry, and perhaps most importantly, Ice Shard. This move is often overlooked due to its pitiful 40 base power, but it's significant enough to threaten KOs on >50% threats/previous threats such as Dragonite, Salamence, Tornadus, Thundurus, Landorus, etc. Ice-type moves are always good, priority is good, and Donphan's base 120 Attack means that with only a little investment you can be threatening tons of KOs from surprising range. And one more thing, Sturdy is a great ability that even prevents a surprise LO Garchomp Draco Meteor or the like from taking you out before you can lay SR or spin. So it almost always gets something done, no matter how insignificant. A nice failsafe, if you will.
Downsides: it possesses a very limited usable movepool, and thus it's predictable. There is little to no doubt about what set it's running. Aside from the odd player who chooses to invest heavily in Attack over Defense, you know exactly what moves it has and how much damage they're going to do. This makes it easy to play around when you see it in team preview unless your team is very specifically weak to it. And most teams run at least 2 mons that it can't touch, or worse, that it's pure setup bait for. Offensive teams will have a Lati, Keldeo, and maybe Breloom or another such bulky Pokemon. Defensive teams will have Skarm/Ferro which can lay hazards at will, and continually threaten a Jellicent switch to prevent spins. Right before BW2 released, some Donphan users were opting for Toxic>Ice Shard, which I think defeats the purpose of using Donphan in the first place, but it performed a very particular job. Additionally, Donphan lacks access to Spikes/Tspikes, making it inferior to Forretress/Tentacruel in that sense. Its spin is harder to pull off because its low SDef and Water/Grass/Ice weaknesses mean it gets one-shot by a ton of stuff. And Sturdy can't always save it, especially if it has to switch in on something or if hazards are already up. Rock resist means SR damage is negated by a turn of Leftovers though. It can't Volt Switch like Forretress either, and it's been replaced in its old role of the Sun archetype's standby hazard protection by one of my personal favorites, Xatu. All in all, as I stated at the beginning of the post, Donphan in BW fills a specific niche that no other Pokemon can do, but it's overshadowed by spinners which are better in 90% of situations. Is it justified in being called OU? I think so. But it's definitely in the bottom 5 of the tier. I'm looking at you, Jolteon.
