Sorry to cause an argument, I really like this thread, but I feel I need to address this-
But that's still just efficiency. You can win any battle with any pokemon with sufficient levelling. How good a pokemon is really just based on how many levels it needs to be raised to win battles. That's why alakazam is better than weedle, because weedle needs to have a much more significant level advantage than alakazam does to win battles.
I also don't see how speed isn't important. What makes alakazam better than weedle is that he can get you through the game much faster. If speed isn't an issue then you might as well say that every pokemon is equal because they can all get you through the game eventually.
That's why I think a tier list is really only relevant for things like nuzlockes that place restrictions on how much you can train (eg. pokecentre limit). But even then, that's still just a measure of what pokemon can win battles with a limited amount of training.
Oh, but efficiency and speed are definitely factors, they are just not the end-all determining factor to see where a Pokemon ends up. We still have Weedle far, far below Abra on our list because we acknowledge that Abra just takes far less effort to beat your average opponent than Weedle does. But we don't want to take this to the extreme of "only use one Pokemon" since it leaves a number of interesting considerations out of the picture and would probably leave this thread without anything to discuss within a week's time.
For example, if you only use one Pokemon, then you can really just dump all your TMs on them and the matter of resource distribution is completely gone. We can't give Diglett credit for learning Dig naturally (which is a great asset), because the Charmander we're solo'ing the game with can just get it at any time.
Also, using one Pokemon will result in that Pokemon being so overleveled that differences between Pokemon become even more rare (which I think you said is a bad thing for discussion overall). Once adequately leveled, you'll just stomp through everything post-Misty pretty much - where's the fun in comparing a bunch of things that OHKO everything? It'll turn to nitpicky things like which one has the most PP.
I guess this will be a game of "let's educate the noob" because I haven't used either Sandshrew or Voltorb. But surely Voltorb needs Thunderbolt way more than Sandshrew needs Dig? Sandshrew learns Slash at level 17 and has an Attack somewhere between Charmeleon and Charizard. It evolves at level 22, giving at a Slash that is more powerful than Charizard's (and Charmeleon evolves much later than Sandshrew does anyway). I don't know, it seems like that would suffice for a while until you reach Earthquake (which I think you can get BEFORE Erika) and it is very unlikely that Dig AND Earthquake would be unavailable to it if you use it on your team. In other words, Sandshrew seems like it would still be a good pokemon even if Dig wasn't available to it.
If Thunderbolt isn't available to Voltorb then it seems like it would be the worst electric type in the game (it is the only one without an electric move of any sort in its level up movepool so it'll be using Tackle and Sonicboom to deal damage for a while), and it would be quite bad until you got to Thunder (and Thunder isn't as good a back-up to Thunderbolt as Earthquake is to Dig).
Or am I missing something obvious?
Nah, I agree with what you're saying, I just thought it was about the closest comparison possible. We should also factor in that Thunderbolt probably has less competition than Dig though, since Dig is in high demand even outside of Ground-types (you'll generally use only one of those), whereas Thunderbolt is only learned by other Electric types and some "movepool wide" Pokemon (Normal types plus Nido family about sums it up), and even in the case of the latter it tends to be just an expansion of versatility rather than needing a raw power move (which means it matters less if they don't get it).