I run a fairly successful Regeneration team on the PO server:
Lineup: Crobat (CB), Mamanbou (Mixed wall), Morobareru (Special wall), Tangela (Physical wall), Tabunne (Special wall / dual screener), Xatu (physically bulky hazard bouncer)
Peaked at 500 on the DW ladder, currently at 323 now that it's been reset. Was at 200 but I stopped playing for a bit.
It works, but it requires a lot of prediction, especially if the opponent is packing Genosect. Reuniclus is a problem. Critical hits can break your defenses and leave you helpless. No hazards hurts.
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That being said, I have tried all of the Regeneration users, and I will state my impressions of each of them
Reuniclus - Use Magic Guard, not Regeneration.
Tabunne - Very good supporter, can take hits surprisingly well and can get in and out a lot and set up screens against a scary foe. The only regenerative user of Heal Bell, so... basically vital to any sort of regeneration team.
Tangela/Tangrowth - Both of these can sponge physical hits very well, but Tangela stands out. People seem very confused when they set up Dragon Dance twice on their
Kingdra to have it do 30% and then have me power swap away their boosts and leech seed them. He can even stand up to Doryuuzu, though with more difficulty.
Morobareru - Holy cow. I use this guy as a special wall, and WOW. Spread status, haze with Clear Smog... it's incredible. Basically, unless it has a stab, super effective attack, it's walled. Even then, he can survive stuff like Espeon's psychic.
Slowbro - He's okay. The psychic typing actually lets him down at times. I didn't use him in-depth because there were so many things that hit him super effectively and he didn't resist that much stuff. He helps, but I ended up preferring Mamanbou
Slowking - Same boat as Slowbro.
Mamanbou - Incredible at taking hits and can pass massive Wishes to team members. You can also run Mirror Coat and devastate opponents who think they can OHKO you with a special move, only to deal 60% or less and then watch you switch back and gain most of your health back. Unfortunately prone to being setup on, especially against Lum berry users who aren't afraid of Boil Over's burn.
Corsola - Surprisingly good. I wouldn't say it's the best, but honestly, it actually helps a lot. I've been in battles where this thing ran the show. It learns Mirror Coat, which combined with a specially defensive spread, can really punish the opponent. It can also learn Stealth Rock, and can spread statuses with Boil Over and Toxic, and if your opponent happens to get sand up for you, it's a great special wall.
Kojondo - Regeneration is good for healing off Life Orb damage and whatnot, but 'jondo isn't going to be taking very many hits. I suppose it's a decent user.