So I just got Emerald off ebay and am going to have a Hoenn adventure to get some Regi's and 3rd Gens in my Unova dex. I've played with Mudkip before on Emerald and once on Pearl so I want to try one of the other two. What would you choose?

Blaziken is way too much of a beast now. I'd pick him if I were you. He's like the ultimate bomber - it's very easy for him to have a moveset with all his moves at 100 or more base power. (Flare Blitz, Hi Jump Kick, Brave Bird, and some filler move.)

I like Torchic, and Blaziken is one hell of a fighter. Especially if you land an Adamant one and ultimately send it to generation 4/5, where its Blaze Kick becomes a physical move and takes advantage of its high Attack stat. Say what you want about Fire/Fighting being an overused starter type combination, it's still a really effective one. If you're looking to just cruise through the game, Torchic will have your back for most of it, knocking out the Team Badguys' Dark-Types and a good chunk of the gyms and Elite Four.

I've never actually used Treecko/Grovyle/Sceptile before. Beyond being a cool-looking lizard thing, and being very fast (Sceptile is the fastest fully-evolved Starter of all) it's not very remarkable. It gains fine moves in the later games... but if you take a good look at any Pokémon's move pool you'll usually be able to figure something out, as well. Sceptile's not bad, but not special. It's garden-variety. Haha.

Personally I like Torchie better. In fact I'm trying to do another solo run with one right now on ruby, but I havent gotten very far recently. I'm even considering not evolving him at all (or evolve him at lv100) and doing the solo run with just that.

Anyway, I always find him easy to grind in the game since it 'seems' like he gets more exp faster. It has a great movepool which gets even better when you transfer him. Oh and I like fire types in general

And a random tip I use: in the first gym, just evolve him and use Double Kick on everything.