Gen 3 Starters the best overall?
With CD moves, they all top their type!
Blaziken: With BB it becomes the best pure Fire attacker, (With both Fire type moves) only let down by certain matchups thanks to its Fighting sub-type.
Swampert: Will be a budget Kyogre with easier dogability and less energy wasted in gyms. It may get more damage off in cases where it has resistances or if Kyogre dies faster.
Sceptile: Will be the undisputed best Grass type attacker. (Unless if the Shaymin formes are any good)
The Gen 1 Starters have Blastoise, (Sucked) the Gen 2 Starters have Meganium (Which is even worse) and Gen 4 has Torterra and Infernape. (They aren't as bad but they won't do too well with their exclusive moves)
Answers
I think there are two strikes going against Sceptile: it's lack of bulk (more minor IMO), and grass typing not being that relevant in the meta. Water attackers are better against rock and ground, electric attackers are better against water. Sceptile tops the DPS chart only if the enemy is a water *and* rock or ground typing (Omastar, Swampert, etc). It's still going to be great and I'm jazzed for all the Gen 3 starters (especially Swampert - guessing it's not getting love because Kyogre exists?), but that's my thoughts on why Blaziken gets more air time.
The one thins sceptile has going for it is clear is a more common weather than rain, in a neutral setting yes water does generally win out if thats not in considered. But the other thing to consider for all of them is they are only bested by legendaries. legendaries mind you, that have much more limited se these days, when it comes to spending RC on saw moltres or Rayquaza/dialga when it comes out, which of the two is more likely to get the candy? likely the dragons because their nonlegendary substitutes are very rare species, other dragons, while for moltres, pokemon like Charizard and blaziken will be much easier to power up for not needing to spend RC on them. Yes in a vacuum the legendaries win out, but when substitutes are easy to get and are similar in preformance, the pokemon with more rare substitutes tend to get the RC. There is also all the good legendaries coming out, people likely would want to save their RC for those legendaries (darkrai, palkia, dialga, shaymin possibly) or for raid day legendaries
Well sceptile wont be the undispured best grass attacker, it will have the best DPS, but Venasaur's poison subtype helps agaisnt things like solar beam groudon extending TDO and a lowing for much more charge moves while sceptile is paper (or rather leaf?) thin, a similar story applied for roserade, having high DPS and the same type combination as Venasaur, giving it more TDO against things like as prior stated SB groudon. Shayming as well does threaten it
Blaziken similar story, double resisting bug like Charizard, but resisting ice like typhlosion, but sporting much more DPS, (seriously regret evolving that 98% combusken) but much like sceptile, its a glass cannon, highest DPS doesnt always make a pokemon the undisputed best attacker, look at deoxys, bulk does play a roll because of practicality.
Swampert however, fantastic, monstrous DPS, great type combination, good TDO, as you said budget kyogre. This seems to be the best of the three, given kyogre's had relatively little competition, but all three wil be fantastic as most legendaries in the future arent weak to these types and only top tier attackers in tier 3-4 raids or raid days/prior legendary returns might be justification enough for powering the legendaries up
