I'm going to talk about all last evolutions of starters, stats, abilities, type-weaks and my opinion about them.
Venusaur -> 80 HP / 82 Atk / 83 Def / 100 Sp.Atk / 100 Sp.Def / 80 Speed
Meganium -> 80 HP / 82 Atk / 100 Def / 83 Sp.Atk / 100 Sp.Def / 80 Speed
Sceptile -> 70 HP / 85 Atk / 65 Def / 105 Sp.Atk / 85 Sp.Def / 120 Speed
Torterra -> 95 HP / 109 Atk / 105 Def / 75 Sp.Atk / 85 Sp.Def / 56 Speed
Serperior -> 75 HP / 75 Atk / 95 Def / 75 Sp.Atk / 05 Sp.Def / 113 Speed
Chesnaught -> 88 HP / 107 Atk / 122 Def / 74 Sp.Atk / 75 Sp.Def / 64 Speed
If you count with Megas, Venusaur and Sceptile have one mega. However they lost the ability to hold items. Mega Sceptile hasn't released yet.
Mega Venusaur -> 80 HP / 100 Atk / 123 Def / 122 Sp.Atk / 120 Sp.Def / 80 Speed
Mega Sceptile -> 70 HP / 110 Atk / 75 Def / 145 Sp.Atk / 85 Sp.Def / 145 Speed
Comparing the stats
I will put Megas and non-megas.
Most HP -> Torterra
Most Attack -> Mega Sceptile, Torterra
Most Defense -> Mega Venusaur, Chesnaught
Most Special Attack -> Mega Sceptile, Sceptile
Most Special Defense -> Mega Venusaur, Venusaur and Meganium (same)
Most Speed -> Mega Sceptile, Sceptile
- Venusaur gets Overgrow that isn't very useful, since he is more a bulky defensive and special defensive Pokémon and Chloropyhll if you invest in a special sweeper.
- Meganium gets Overgrow, depending on your set this can be useful and Leaf Guard that is the same as a Safe Guard but you need to get a Sun Day or Drought.
- Sceptile gets Overgrow, great ability for him, makes him a great sweeper and Unburden is a perfect ability here! Doubles his speeds and nothing can outspeed him ever with scarf (some still can) and setting a Swords Dance for sweep with a strong moves.
- Torterra gets Overgrow but his speedis low and Shell Armor can't make criticals in this Pokémon, still has a x4 weak to ice-moves.
- Serperior gets Overgrow, great speed can outspeed with a 1.5x from this ability but you need survive the move with low HP, something rare and Contrary each hasn't been released that makes Leaf Storm very very strong making Sp.Atk to boost and having a Timid on this, ouch!
- Chesnaught gets Overgrow, has great defense for this but not special defense. Bulletproof isn't the best ability but can protect you for some attacks such as Focus Blast, Sludge Bomb..
- Mega Venusaur gets Trick Fat and makes hit bulky to all fire and ice types.
- Mega Sceptile gets Lighting Rod and you can use it for switch your water-type to mGea Sceptile and get a Special Attack boost, very useful ability for this situations.
Types and weaks
Venusaur is a Grass/Poison type and is weak to -> Fire/Ice/Psychic/Flying.
Meganium is a Grass type and is weak to -> Fire/Ice/Flying/Poison/Bug.
Sceptile is a Grass type and is weak to -> Fire/Ice/Flying/Poison/Bug.
Torterra is a Grass/Ground type and is weak to -> Fire/Ice/Flying/Bug.
Serperior is a Grass type and is weak to -> Fire/Ice/Flying/Poison/Bug.
Chesnaught is a Grass/Fighting type and is weak to -> Fire/Ice/Flying/Poison/Psychic/Fairy.
Venusaur and Torterrahave the low weaks (4) and Chesnaught has the more weaks (6).
Opinion from best to worst
1st -> Sceptile (Fast sweeper with Swords Dance and Unburden strategy)
2nd -> Venusaur (Great Wall with recovery moves and Toxic, perfect ability for mega)
3th -> Chesnaught (Spiky Shield and great Attack and Defense)
4th -> Torterra (Powerful STAB Earthquake but weak to a ice-type)
5th -> Serperior (Leaf Storm + Contrary, but Contrary hasn't released yet, Overgrow isn't so great ability so Leaf Storm will make Sp.Atk weaker every turn)
6th -> Meganium (Don't like it so much, same role has Venusaur but the ability don't fix there..)
Hope this helps.