Mudkip has the highest potential to be able to build a great movepool in-game without breeding it or using the Move Tutor to teach him moves. Swampert is also the best out of all the thirteen starters.
Torchic still has potential in-game, provided you know what you're doing with him. The only issue is that Rock Slide is not available in Ruby unless you breed. The Move Tutor that teaches Rock Slide is only in Emerald, and Rock Slide is far more useful on Blaziken than Earthquake, which is overused on him and is better on someone else. Just get rid of Blaze Kick - it sucks.
Treeko has very limited potential in-game unless you're seriously considering breeding. Leaf Blade is a brilliant move, but it's the only good move Sceptile learns by level-up, and the only good TMs it can learn are Physical ones (with the exception of Dragon Claw, which is nothing special). Sceptile is a damn good pokemon is arguably a step above Blaziken if his full potential is reached, but it's so time consuming to bring it that you'll probably be happier with one of the other two instead.
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