I'm answering this, since I have a few things to add (and the grammar of the first answer kind of sucks).
Yes, the Gen 4 starters are the most balanced. In fact, Empoleon's lack of a strength against Torterra helps balance them further, for these reasons:
- Empoleon is part steel, so if it had coverage against Torterra it would be the most powerful starter.
- Grass types are terrible in general, so Torterra's advantage over Empoleon helps it out.
And the starters for all of the other generations are unbalanced.
Gen I: Very unbalanced, since Charizard (Fire/Flying) has a dual strength over Venusaur (Grass/Poison)
Gen II: Standard Fire/Water/Grass starters.
Gen III: Swampert has a dual strength against Blaziken and a dual weakness to Sceptile.
Gen V: Standard Fire/Water/Grass (albeit Emboar's part Fighting type)
Gen VI: Completely unbalanced, since their types are Grass/Fighting, Fire/Psychic, and Water/Dark.
However, in a way, the starters for all generations are balanced, due to the fact that Grass beats Water, Water beats Fire, and Fire beats Grass. Of course, they aren't balanced in the way you mean (i.e. no other generation has a Fire type with a STAB move which is super effective against the Water type of that generation, etc.)
I hope this answered your question.