HP Ground or Fighting to get a Magneton switch-in. Preferably Ground. And Surf over Psychic? With Vileplume and Poliwrath in the tier? You must be on shrooms.
First of all, cut out the condescension-filled posts, they are getting old.
Second, please understand how STAB and Base Power works. If you run HP Fighting, you're going to hit Magneton for 140 Base Power...while STAB Surf will hit it with 142.5 Base Power. You need to also understand how coverage works. You want Surf and Psychic as your attacks because they get STAB and have near perfect neutral coverage (except on opposing Slowking, Sharpedo, and Shiftry). With Hidden Power or Ice Beam you lose a ton of coverage on many threats (specifically Fire...).
Additionally, you need to make sure you are hitting everything you need to hit. Yes, Sharpedo and Shiftry are dangerous....before you hit them with Thunder Wave. That's what that move is there for. Without Surf, you're going to get your ass handed to you by every single Fire-type. Magmortar will have field day on your team because you tried to surprise OHKO instead of settling for the ability to severely cripple it.
You also need Defense on Slowking so that you aren't destroyed by physical attackers, specifically Medicham.
In conclusion, Surf / Psychic / Thunder Wave / Slack Off will probably be the best moveset. I use a mixed Defense / Special Defense EV spread to survive specific attacks (like LO Adamant Head Smash from Relicanth).
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My main issue with this thread is that people are talking about Regirock / Slowking / ....Vileplume..? Why Vileplume over Meganium? I know it has that fancy move called Sleep Powder, but that isn't worth losing severely to many threats. Without Meganium you're going to get dominated by Relicanth, Rhydon, SD Gligar, SD Sandslash, <insert Pokemon with Earthquake or an unresisted physical STAB>. Meganium checks
all of those threats and then some. What does that added Poison-typing get you? You check....opposing Grass-types better? Everything else Poison would help with is checked by Slowking or Regirock. It is definitely not worth losing an Earthquake resist / physical mon check.
Grass-types are fodder in this tier for the excess of Fire-types and Haunter. All you need is something to beat opposing LO Vileplume and you're really set against everything Vileplume would give you over Meganium.