Feraligatr in Silver for my first playthrough. It wasn't even a solo starter run, but played so much of that I lost count of all my hours, though I think I also leveled up some of my other mons to 100 or at least very close.
I'd like to see a region based on Germany next, since Gamefreak went with a European country this gen. Since Kalos (France) is nearby, we could potentially see two regions like Johto-Kanto again. Maybe even give the Johto starters with mega stones.
I'd like to see a Ground type eevolution with a spread of 65/130/95/60/65/110 or 65/130/110/60/65/95. It could be a fast sweeper or bulky (on the physical side) attacker. Something like Swords Dance/Earthquake/Stone Edge/Knock Off would be amazing.
Who's more useful? Feraligatr, due to Sheer Force. Swampert's mega is completely dependent on rain.
Who'd win in a fight? Tough call, aesthetically regular Gatr could take on regular Swampert, but have a hard time against the muscle bound mega. 1 on 1 gameplay wise Feraligatr due to Water being...