You can do Johto with an unevolved Zubat. But there are easier ways. Just because you beat the game with one unevolved doesn't mean squat if you're talking about the better Pokemon. Of course, it's doubtful that the person will be making a solo run at it, meaning that team considerations should also be involved.
Gyarados in game is much better than Totodile, and you receive one via the Shiny encounter without ever having to raise a Magikarp. He outclasses Totodile both in moveset and Feraligatr in stats and ability. And even if you don't want to go that route, there are countless other water types that have a place on a team. (Slowbro, Golduck, Poliwrath, Kingdra, Lapras, Azumarill can all be used to success)
Typhlosion on the other hand offers the player a way to obtain a new Fire Pokemon (otherwise having to obtain one of the Kanto fire types), with Slugma and Houndor not available until Kanto.
And if you really want to get into the 'better Pokemon' in game, speed is a gigantic matter. Cyndaquil line outspeeds Totodile line handily, and in HGSS, it has access to Fire Blast right off the bat. Feraligatr won't receive Surf until later, working off his weaker SpAtk, or Waterfall MUCH later (around the time he'll learn Aqua Tail naturally). And even pure power wise with move aside, Typhlosion has higher base SpAtk than Feraligatr doesn't attack.
So there.
