Swampert has the highest base stat total of all starters to this day and the fewest weaknesses. Mudkip has the most physical attack of first form starters. There are no grass gym leaders or elite four members in Hoenn. Lack of recovery isn't a problem in single player because potions. Has good stats all around except for speed, so less likely to get screwed by bad natures. STAB Surf and Earthquake are great for all the double battles. Bodies Roxanne, Wattson, and Flannery single handedly if you're not underleveled, at which point you should have a decent team together. STAB Earthquake, a spread move with 100 BP and no repercussions besides hitting teammate in doubles. Ice Beam/Blizzard coverage and able to use Rock Tomb from first gym for EdgeQuake coverage before Brawly. Rain mega in routes with constant rain. Actually fine with Waterfall+Surf in ORAS, and fine with Strength, reducing need for HM slave. Counter, Mirror Coat, Stealth Rock, Roar, Hammer Arm, Body Slam, Superpower, Earth Power, Scald, Power-Up Punch, Curse, Barrier, Ice Punch, Ice Beam, and Avalanche. Reason to use both normal and mega, as opposed to the original being outclassed. Was good before gen 5 and is the second highest tier-wise(non-Mega Sceptile's NU), while being the best for hoenn as described before. Has variety to its movesets and has been consistently good regardless of which game you're playing. No shilling from GameFreak. The line has the best meme material(you're on 4chan, btw). And Finally, has a more creative real life animal basis in the mudpuppy/axolotl.
Swampert is the best, no question.