Blaziken / Sceptile / Swampert
Pros: Blaziken has the ability to sweep the game with his greatly gifted offensive stats and comes in very handy against the Elite 4. He is a Fire/Fighting type duo with few weaknesses. Blaziken has great speed and can outspeed most of the ingame Pokemon.
Cons: Blaziken has horrible defensive stats and isn't bulky enough to tank out an occasional Earthquake or Earth Power from the opponent. It struggles near the start if not evolved into Combusken by the first Gym.
Pros: Sceptile has the best speed out of all three Hoenn starters and has to be one of the fastest Grass types still. It has decent offensive stats and learns some great moves with level up and TM. Its movepool provides amazing coverage throughout the game.
Cons: Sceptile's only cons are that all its good STAB's and coverage moves are learnt at higher levels in the game, and has to rely on Razor Leaf and Pursuit to carry it swiftly throughout the game. It is also a pretty frail Pokemon by Base Stats.
Pros: Swampert's great typing and extensively large natural defences make it a great staller, even Ingame. It has decent HP and great Attack power to bore through the game. The moves it learns are also pretty nice.
Cons: Swampert's speed is its main downfall and stops it from becoming a five star sweeper with equally monstrous defences. It will outspeed many Pokemon ingame but will get outsped by many many more.
Blaziken: Pure attacking Power and sweeping.
Sceptile: Great Speed and decent offences.
Swampert: Great tanking and defences.
I have chosen all three starters for RSE and have a full game experience of them all. My best game was with Blaziken and the Worst was Swampert. I failed completely with Swampert and couldn't even beat the Elite 4 no matter how hard I tried. Blaziken allowed me to sweep the game in one week. Sceptile was challenging and probably the most fun game but it wasn't like Swampert where I totally failed.
Hope I helped! :)