Well it really depends on what you want to do. I wouldn't pick Blastoise or any of the Water type starters because there are a ton of good, better Water types out there. So its really between Charizard and Venasaur. Now it is very important to have a balanced team at all times to fend of all your weaknesses and I always build that out of a Fire type, a Water type, a Grass or Electric type, a Flying type,a Phychic type, and personally a Ground type. So lets look at those 2 Pokemons Stats and Movepools.
Venasaur
Stats
HP - 80
Attack - 82
Defence - 83
Special Attack - 100
Special Defence - 100
Speed - 80
Movepool
Start - Tackle
4 - Growl
7 - Leech Seed
10 - Vine Whip
15 - Sleep Powder
15 - Poison Powder
Evolves Into Ivysaur
22 - Razor Leaf
29 - Sweet Scent
Evolves Into Venusaur
38 - Growth
47 - Synthesis
56 - Solar Beam
Ability - Overgrow - If this Pokemon is bellow 1/3 of its Hp, the Power of Grass type moves will increase by 1.5x
My Venasaurs moveset is going to have
Synthisis
Solar Beam
Sleep Powder
Razor Leaf
Now lets look at Charizard
Stats
HP - 78
Attack - 84
Defence - 78
Special Attack - 109
Special Defence - 85
Speed - 100
Movepool
Start - Scratch
Start - Growl
7 - Ember
13 - Metal Claw
Evolves Into Chameleon
20 - Smokescreen
27 - Flamethrower
Evolves into Charizard
36 - Wing Attack
44 - Slash
54 - Dragon Rage
64 - Fire Spin
Ability
Blaze - If this Pokemon is bellow 1/3 of its Hp, the Power of Fire type moves will increase by 1.5x
Recomended Moveset
Flamethrower
Sunny Day or Fire Blast
Wing Attack
Steel Wing
Venusaur is the best in the fact that it can get through alot of the early Gyms easily. I think Charizard is the best of the two. It was a close call because Venasaur is very good. But I think why Venasaur lost was because it has kind of a shallow movepool.
Dont be discouraged on using Blastoise, Its just I think there are better Water types out there. Namely Gyrados, Lapras, and Vaporeon.