What does a signature move have to do with which Poke a person should choose? A good trainer will pick the one that is most effective. A STAB Earthquake from Swampert does more damage to more Pokes than anything the other two starters can do.
As for your attempt to make a case for Blaziken in the Weak5, you are not taking into account the following:
- Against Sidney, Swampert can handle all of them using IB against the Grass-types, and EQ against the others.
- Against Phoebe, Swampert sweeps through using EQ.
- Against Glacia, Swampert has a tougher time but EQ still works.
- Drake is a no-brainer.
- You also battle Wallace in EMERALD, not Steven, although Swampert also does very well against Steven, moreso than Wallace.
The fact is that Blaziken and Sceptile both run into big time problems with certain gyms and certain Weak5 members, while Swampert rolls through everything. Plus, as I said before, Swampert can achieve its most effective moveset (Earthquake/Ice Beam/Rest or Protect/Surf) BEFORE the Weak5, both Blaziken (Flamethrower or Fire Blast/Rock Slide/Sky Uppercut/Swords Dance) and Sceptile (Leaf Blade/Leech Seed/Substitute/Thunderpunch) have to wait until you get to the Battle Frontier and earn enough Battle Points to get theirs. Then there is the typing. Swampert easily has the best typing (Water/Ground) that negates one of Water-type's biggest weaknesses (Electric) and also learns a terific move to cover its only weakness (Ice Beam vs. Grass-types). Blaziken is weak to Flying/Ground/Psychic/Water (ouch!) and Sceptile is weak to Bug/Fire/Flying/Ice/Poison (again, ouch!). Not to mention the fact that Swampert is strong enough defensively to be a decent tank and take a few hits.
As I said before, people can choose Treecko or Torchic because they look cool or want a challenge, but for the means of picking the best and most effective starter (especially in-game), it is Swampert.