I would say Mudkip is the easiest starter to train, mainly because type resistances are not as much of a concern for Swampert compared to Blaziken and Sceptile. In general you have a much higher chance to hit a Pokémon for at least neutral damage with either a Water-type attack or a Ground-type attack--this is especially important for beginning trainers who may not completely understand the type chart, because they're much better off spamming a Water- or Ground-type attack against a Pokémon they don't know the type(s) for and still do damage. While Blaziken's Fire- & Fighting-type attacks also have decent coverage, there are more resistances to take into account with these two types, so using a Blaziken requires a bit more knowledge about Pokémon types in order to be successful. Unfortunately the Grass type is known for being the most resisted type in the game (tied with the Bug type), so it's not a good type for beginners or for quick training, in my opinion.

In terms of most useful starter, I would say that Mudkip also takes the cake, mostly because of HMs. You can't explore half of the region without having a Surfer, and the Mudkip family is the only Hoenn starter family that can learn it. Dive and Waterfall spots are less prominent in Hoenn, but it's important to note that you can't get to Hoenn's Pokémon League without having a Pokémon that can use Waterfall, an HM that, again, only Mudkip can learn. Mudkip can't learn Cut--somthing Treecko and Torchic can--but from what I remember it's never required in Hoenn to continue the story, unlike in, say, the Unova region. (Lemme know if I'm wrong!) And as for other HMs required for story progression (Strength & Rock Smash), all three starters can learn these attacks. (Treecko is the only starter that can use Flash, but you can still technically traverse through pitch black areas without using it. None of the starters learn Fly, but it's never required to Fly in any game iirc.) This makes Mudkip a "Swiss army knife" of sorts, since it has the ability to learn so many HMs in case you can't/don't want to teach these attacks to another Pokémon. Oddly enough, this is Mudkip's main disadvantage as well, since it suffers from "four-move-syndrome", but at least the potential to learn so many HMs is there.

Competitively speaking, Torchic has it hands down. Not only does it have amazing sweeping stats, but with the Speed Boost ability, Blaziken can take its biggest problem as a sweeper and fix that problem up in one or two turns. But what's important to remember is that Blaziken is a monster even with the Blaze ability, 'cause if you're able to keep your Blaziken's HP in that Blaze-activation zone without making it faint, you're getting that boost on top of high attacking stats and access to three of the most powerful moves in the game--Flare Blitz, Brave Bird, and High Jump Kick. Granted these attacks have major drawbacks (damn recoil) but that raw attacking power is nothing to scoff at. Swampert and Sceptile simply cannot compete.

Personally speaking, I love all three Hoenn starters because they're the most memorable starters in my opinion (outside of the Kanto starters). My personal favorite is Mudkip, but I wouldn't mind using any of them in a play through!