That Milotic gets a big thumbs down, sorry... half the time you will be wasting your turns using mirror coat because they won't attack you, and anything with substitute, probably one of the most common moves in UU, just uses you as complete set up fodder. Sub-Seed Venusaur, Sub-Plot Mismagius, Sub Punch Poliwrath are just a few of the common pokemon that can rape your team after they easily get a sub up.
I'd suggest a standard wall Milotic like...
Milotic @ Leftovers
Bold Nature... +Defense, -Attack
248 HP / 252 Def / 8 Speed
~Surf
~Ice Beam
~HP [Grass]/Toxic
~Recover
You can pick if you want HP [Grass] for Sub Dance Gatr, Sub Punch Azumarril etc... or Toxic for Slowking, other Milotic etc...
Careful is the nature to go on Regirock. He has enough defense, so to be honest, no EVs at all are needed in defense. Give it the standard EV spread of 252 HP / 220 Special Defense / 36 Attack which makes its defensive stats 364 HP, 436 Defense, and 320 Special Defense - more than enough for any wall. You want Explosion in slot three and Rock Slide in slot 4.
BTW Drapion isn't NU, he is UU. He is a great Swords Dance Sweeper, Pursuit Trapper, Utility Pokemon etc... and is amazing in UU
Choice Scarf Venusaur actually isn't that uncommon, so don't be unsure of its ability. Most of the time it is seen as a lead with Sleep Powder, but the way you have it is nice, too.
Good Toxicroak set.
Arcanine's physical set is much more productive than its special set to be completely honest.
Arcanine @ Life Orb/Leftovers
Adamant Nature... +Attack, -Sp. Attack
120 HP / 152 Attack / 236 Speed
Intimidate
~Flare Blitz
~Extremespeed
~Morning Sun
~Will-O-Wisp/Toxic
The EVs allow you to outspeed standard Jolly Venusaur, have good bulk, and provide strong attack. Life Orb for power... Leftovers for recovery. With Flare Blitz and SR taking your HP down, you may not want life orb, but that is why Morning Sun is there.
Toxic is for all those bulky waters that switch in on you, and WoW is a great move to incapacitate physical sweepers and use their now 1/2 attack power to set up with another pokemon (like Toxicroak)