You don’t even really need a Grass type there, so you could settle for a powerful starter and *maybe* a mediocre grass type which again, can play second fiddle. You also1. "Best Pokemon of its type" is more of an issue in a game where said type is actually important, like Fire in Sinnoh, where Infernape gets a lot of merit since it makes your job a lot easier. In Johto you won’t lose much by using a different grass type, but in Sinnoh, you'll lose out on a lot if you don’t chose Infernape. This rings specially true when Meganium has almost no coverage. Probably the only helpful support is screen set up that and that usually isn’t needed in playthroughs, and there are options for that.
The "best starter" merits are usually based on: 2. is it powerful on its own? 3. Does it play an integral part in the team? 4. How does it work with the team? 5. Type advantages against the gyms etc., so if someone modifies the criteria enough one can probably crown any starter the "best one".
Ok before I respond to the points you raised (which I added numbers to for easier clarity of discussion), I just want to point out that No type is needed, Pokemon is an easy game with a level system, and folks have beaten games with a single pokemon with 1 hp, among a myriad of other self imposed challenges. You can solo the game with a Wheezing if you are so inclined, it's actually a very good Pokemon, it just never sees use or consideration because folks naturally build their team around their starter, which is self imposed tradition and laziness more than meritous team building tactics, but it is what it is. It certainly works, though it's not the only point of reference to work from. Just saying, you don't "need" any given Pokemon, of Any type.
1. It's not an "issue", it's just something of note, a consideration. If I want a good grass type, this is by far my best option. If I want to build a team with a given coverage set and grass is a complementary piece of that (for example, a strong grass type nearly perfectly complements a strong fire type), I'm far better off long term with say Entei and Meganium than I am with Typhlosion and Bellossom, would you not agree? Again not at all required to beat the game, but as far as enabling different options for teambuilding, it's a strong and difficult to replace piece.
2. Though I'm specifically approaching it from a different angle than that, I did also point out that if you're looking for strongest on its own, IE strongest Solo Pokemon, Totodile is the best starter. However why would you need the strongest Pokemon on its own? Quagsire is arguably better suited for that task than Totodile is, and it's not inherently a solo game.
3. No pokemon plays an integral role on any team, not necessarily at least. Wheezing and Houndoom can partner up and take all comers just fine. Anything Typhlosion can do, Entei can do better. Anything Feraligatr can do, Suicune can do better. So how "integral" can they really be?
4. That is exactly why having the best Pokemon of a given type is great, because then it works best with the team. I can get through the game just fine (and am, nearly done) with a "team" of Typhlosion, Quagsire, and Weepinbell, plus 3 HM slaves. But for situations where Grass types are useful (against ground types and water types specifically, since the rest of my team is neutral or worse against them) I would *love* a better grass type, and Typhlosion is on my team while Entei sits in a box solely because I like most folks am going to use my starter through the whole game, even when it's not the best option. It's not a wise, rational, or strategic decision, and I really should replace my starter, because Entei would work better with my team. Something that would not happen if I was using Chikorita from the start.
5. Type advantages against the gyms are nice, but you can pick up a Pidgey or Hoothoot first thing, and they will have more advantageous matchups than Any of the starters will through the first 5 gyms, and by the time you reach gym 6, you have access to Pokemon of every type, so if you want a fire or water type you can have your pick of the litter. It's just not that important, especially because your starter is rarely the best option, and certainly isn't the only option.
Just my opinion of course, but I will point out that from every standpoint you list, Pidgey outclasses every starter, and Chikorita is the best complement to a Pidgey opener. Pidgey is weak vs Electric, Rock, and Ice, and neutral vs Water, all of which other than Ice are helped by a strong grass type. Ice, meanwhile, isn't common until late game, when you have plenty of answers for it. Waters advantages vs ground are redundant with flying's advantages, same with fire vs grass and bug. So in the early game, the most important time in the game, a team of Pidgey and Chickorita fares better than any other early Pokemon with any other starter, does it not? That combined with the fact that Chikorita has no reason to be replaced late game sure seems like a meritous starting point for an analysis of best starter to me!
