This is random, but was I the only person who was unaware that it is actually possible to obtain 5 pokeballs from Professor Oak in RBY?!? If you beat your rival on route 22 and then backtrack and talk to Oak without buying any pokeballs or catching any pokemon, he will give you 5 pokeballs! And I thought I knew everything about this game...
I'm thinking from my current playthrough that Yellow Squirtle is good enough for High. I was a little concerned about the very low level it came at, but it is very easy for it to catch up. I babied it against the trainers east of Vermillion (it was even able to beat a few things on its own). It was at level 18 when all the trainers were beaten. Then I just went to Diglett's Cave and took it to level 22, at which point I decided that level 22 was good enough. The whole process of going from level 10 to level 22 took around 30 minutes. I continued my journey by going east of Cerulean, where Wartortle proved to be very useful against the large number of Ground types there (it also held its own against the weak normal types like Rattata). Its usefulness continued through Rock Tunnel, and from there it just plays like RB Squirtle.
It isn't top because it does come at a very low level, but given how easy it is to grind and how useful it proves to be along the upcoming routes (and the game in general), High seems like a good fit.
Also, in regards to my quest for Porygon, I have been spending as little money as possible so I can get it as soon as I can (I have bought 1 pokeball total which I was able to get away with thanks to discovering that Oak can give you pokeballs). I have also been selling anything that I didn't absolutely need. This includes basic stuff like Potions but also things like Vitamins and even TMs that I wan't going to use, like Bide and Seismic Toss! I'm at Lavender and I have enough for about 5000 coins, so I'm probably NOT going to be able to get it as soon as I reach Celadon (not that I was expecting to). Not that it matters; Porygon is obviously going to be Low tier (I'm only using it so I can get an idea of its battling strengths, though I'm not expecting much).