Venusaur has above-average bulk and a good support movepool; its offensive choices are somewhat limited, however. Still, the ability to spread status is always nice, and Venusaur can put opponents to sleep or poison them (with either Sludge Bomb or Toxic), so with the support of a counter to Celebi and Blissey, such as Hidden Power Bug Dugtrio, Venusaur can effectively cripple a wide range of threats and prove to be a general nuisance for the opponent.
Sludge Bomb is the third option here for a reason—the aim of this set is not to sweep or deal heavy damage to opponents. Venusaur will probably be met straight up by Skarmory, if not Celebi, so Sleep Powder should be its first move, followed by Leech Seed. It can then wear down the opponent with Sludge Bomb, healing when necessary with Synthesis. You should note, however, that sandstorm reduces Synthesis's recovery from 50% to a measly 25%.
The EV spread is not set in stone, but all variants should run 252 HP EVs for maximum tanking ability and a few EVs in Speed. The rest is split between Defense and Special Defense depending on the threats that you wish Venusaur to wall. Venusaur's Special Defense is quite a bit better than its Defense, and it has more special resistances too, so it works very well as a special tank. With a Careful nature and heavy investment in Special Defense, this set is good at walling even Hidden Power Ice variants of Zapdos and Raikou for quite a while. However, if you would rather Venusaur take on physical threats such as Fighting-types or Swampert, focus on Defense instead.
This might not really be a good idea with the ubiquity of Skarmory, but it can be surprisingly effective with Magneton support. Sludge Bomb is Venusaur's physical STAB, and deals heavy damage after a boost or two. Earthquake covers Metagross and Tyranitar, and if you're afraid of Ghost-types, run Hidden Power Ghost over Sleep Powder in the last moveslot.
108 Speed EVs enable Venusaur to outspeed neutral-natured Tyranitar, while the 208 HP EVs give it a healthy 353 HP to make setting up easier. The rest of the EVs are dumped in Attack, and in conjunction with an Adamant nature, this gives Venusaur sufficient firepower to pull off a sweep. If you're going with Curse, remove all Speed EVs, maximize HP, and put the leftover EVs into one of its defenses. Defense investment enables Venusaur to set up Curse boosts with ease on physical attackers such as Snorlax, while Special Defense EVs let Venusaur take special hits well while boosting itself on the physical side.
Venusaur can also run a specially based set to surprise counters such as Skarmory. This set can check SkarmBliss cores with ease—Skarmory is taken out by Hidden Power Fire, while Blissey is put to sleep and stalled out with Sleep Powder and Leech Seed. Venusaur is walled by threats such as Salamence, but once they are removed by a teammate, it can pick its way through the opponent's team.
If Blissey is their only decent counter to Venusaur, you can put it to sleep with Sleep Powder with some prediction. Assume a situation in which Venusaur is facing down Blissey, and the opponent has Swampert and Skarmory waiting in the wings. If Venusaur puts Skarmory to sleep, Blissey can come in and stall Venusaur out of PP with Softboiled, go for the freeze with Ice Beam, or paralyze it with Thunder Wave. If Blissey is put to sleep, however, it might switch out to heal off the status with Natural Cure, and then come right back in. In that case, reuse your sleep move, as Natural Cure means you won't be violating Sleep Clause. You can do the same thing to potentially beat Celebi, but you have to correctly predict Celebi switch-ins; otherwise, it can hurt Venusaur with a faster Psychic.
Hidden Power Ice can beat Salamence on switches, but Venusaur needs at least 270 Special Attack (136 EVs without a boosting nature) to even have a shot at OHKOing the standard 4/0 Choice Band Salamence; 319 Special Attack—216 EVs and a boosting nature—is needed for the guaranteed OHKO. With maximum Special Attack investment, Venusaur has a 67% chance to OHKO the standard 120/0 Dragon Dance Salamence. Hidden Power Grass is a bit stronger than Razor Leaf and has 100% accuracy, but it's not usually worth sacrificing the high critical hit rate and a Hidden Power slot. Body Slam can paralyze Salamence, among others. Don't even think about Sunny Day + SolarBeam, however; Venusaur is too slow to set it up, and is outclassed by other Chlorophyll Pokemon, such as Exeggutor. Similarly, Growth can be used to boost Venusaur's mediocre Special Attack, but such a set is better done by Calm Mind Celebi.
Light Screen can be used to help the whole team take special hits more easily. Roar can force out Substitute, Agility, or Baton Pass Zapdos, and is another alternative to stopping Calm Mind Raikou. It also defeats Ninjask, but watch out for Hidden Power Flying. Charm can annoy the likes of Salamence, Gyarados, Snorlax, and Swampert. Protect scouts Explosion and Choice Band moves, and, more importantly, gives Venusaur more Leech Seed and Leftovers recovery. Toxic is a decent move for stall-oriented teams, and can pair nicely with Leech Seed. Just make sure you have a way to stop Skarmory and Celebi—consider Sludge Bomb and Hidden Power Fire, respectively.
Checks and Counters
Skarmory walls any set that lacks Hidden Power Fire. Gyarados and Salamence can tank all Venusaur's attacks bar odd Hidden Powers. Celebi sops up Leech Seed nicely, resists Grass-type attacks, and laughs at anything offensive Venusaur throws at it, save Sludge Bomb. Nonetheless, all of the above have STAB super effective attacks to put a hurting on Venusaur. Gengar, Weezing, Forretress, and Metagross resist or are immune to both Venusaur's STAB moves, but Venusaur can still put them to sleep or stall them with Leech Seed; Metagross also needs to watch out for Earthquake and Hidden Power Fire. Tentacruel makes Venusaur's life hard with Liquid Ooze and Substitute to thwart Leech Seed, and packs either super effective Ice Beam or STAB Swords Dance-boosted Sludge Bomb; however, Earthquake can take it down.
Blissey hates Leech Seed and Sleep Powder, but soaks up all special attacks and easily survives two Sludge Bombs. Snorlax beats Venusaur in a Curse war, unless the latter is carrying Leech Seed. Regice and Articuno don't particularly care for Hidden Power Fire and can fire off STAB Ice Beams, so Venusaur can't stay in to use Sludge Bomb is risky.
Most counters need to be wary of switching into Venusaur, as Sleep Powder can put even the sturdiest foe out of commission if they are not prepared for it. However, Hypno and Banette don't take much damage from Venusaur's STAB attacks and are immune to Sleep Powder. The former can deal heavy damage with STAB Psychic, while the latter can burn Venusaur with Will-O-Wisp and remove its item with Knock Off. Heracross and Ursaring actually benefit from Venusaur's Sleep Powder thanks to Guts, and can take advantage of this with Sleep Talk and a powerful STAB move.