Even from the very first Pokémon games, Pokémon that were worth investing in early in the game were few and far between, though that has changed over the years with Game Freak realizing that players don't want to replace members of their team after a few gym badges have been collected.
When it comes to Pokémon Red & Blue, the best Pokémon can be easy to overlook in favor of later catches like Tauros, Eevee's evolutions, or the all-powerful Snorlax. That doesn't mean there aren't some decent early game options that are worth a trainer's time to invest in them, though not all are created equal.
10 Invest: Spearow
For early game Flying-type Pokémon, most players will opt for Pidgey to carry them through the game, as Ash's iconic Pidgeotto, and later Pidgeot, is one of his most memorable Pokémon from the anime. However, Spearow and its evolution, Fearow, are better at doing what Pidgey does in virtually every way. Aside from being both faster and more powerful, it reaches its final evolution much faster at a mere level 20 as well it learning Drill Peck, an 80 base power Flying-type attack that is significantly better than anything Pidgeot can learn, and doesn't require one turn of charge like Fly or Sky Attack.
9 Leave Alone: Caterpie/Weedle
Like Pidgey, many players will catch a Caterpie as an homage to Ash's original team, as Butterfree holds a special place in many people's hearts. While Butterfree can be very useful against Brock, particularly if players chose Charmander as their starter, it quickly begins to lose its usefulness and isn't worth the difficult grinding required to evolve it before the first gym. In general, Pokémon Red & Blue doesn't treat its Bug-type Pokémon very well, with even the likes of Scyther not being very good until several games after its initiation to the series. Even Beedrill, one of the few Pokémon in the game who can learn a damaging Bug-type attack, is quickly overshadowed like its butterfly counterpart and isn't worth the time and energy to evolve it.
8 Invest: Nidoran
One of the few options players get to make the battle against Brock a little easier in Pokémon Yellow is Nidoran, which learns Double Kick at level 12 and can easily mop the floor with the Rock-type gym leader by itself. Unfortunately, its Red & Blue counterpart doesn't learn this move until level 43, but that doesn't mean it isn't worth catching.
Evolving into Nidorina or Nidorino at level 16, it's very easy to have one evolve on the way through Mt. Moon, which is useful because players can find a Moon Stone inside this dungeon to evolve it to its final form before they face down the Fossil hoarder. Although this heavily limits the moves it will learn by level up, this doesn't matter much as the best moves they can learn are all from TM's.
7 Leave Alone: Pikachu
This might be considered blasphemy, but Pikachu is not a good Pokémon to invest in, at least in the original games. Aside from being annoyingly hard to find and catch in Viridian Forest, it is also incredibly weak and can't be evolved until players reach Celadon City so they can buy a Thunder Stone. At this point, the gift Eevee also becomes available, which means Jolteon, the best Electric-type in generation one, also becomes available.
6 Invest: Magikarp
Magikarp has a long-standing reputation as being one of the hardest Pokémon to level up, with the best reward when it finally reaches level 20 and evolves into Gyarados. The Magikarp salesman in the Pokémon Centre before Mt. Moon may be played off as a scam artist, but for a mere $500, players can potentially face down Misty with a powerful Water-type of their own. Since generation one hadn't split the special stat into attack and defense, this also means that the Bubblebeam TM won from Misty is actually useful on Gyarados, whose special stat is a very impressive 100 and can make full use of the Water-type move.
5 Leave Alone: Pidgey
As mentioned earlier, the best early game Flying-type is easily Spearow, despite it often being played off as the weaker of the two in the anime. If players are playing Pokémon Yellow, it could be worth searching high and low for the elusive Pidgeotto in Viridian Forest, but even then, it's best to just catch a Spearow and wait a little longer for it to evolve. The Pidgey line also has fairly bad stats for the levels it evolves at, as it doesn't fully evolve until level 36, the same as the starter Pokémon, all of which have better stats. Charizard is even better at being a Flying-type.
4 Invest: Abra
Abra may be a pain in the butt to nail down and catch, but if players are in need of a destructive Psychic-type Pokémon, look no further than Abra and its evolutions. Abra, like Magikarp, is a little bit annoying to try and evolve, but has the advantage of being able to learn TMs and evolving at level 16 rather than level 20.
Even if players don't have the ability to trade evolve it into an Alakazam, Kadabra is quite powerful on its own and can hold its ground against even fully evolved Pokémon later in the game. The only real downside to the Abra line is its atrocious defenses, but for Pokémon as fast and powerful as these ones, it doesn't matter too much in the long run.
3 Leave Alone: Jigglypuff
Even now that it has the Fairy-typing, the Jigglypuff line isn't overly useful for what it does, as its counterpart, Clefairy, does the same job better. Because it evolves via Moon Stone like Nidoran, it can be tempting to catch and evolve one before exiting Mt. Moon, but this is a giant waste of a Moon Stone. Unlike Nidoking and Nidoqueen, Wigglytuff doesn't continue to learn moves after it has evolved, meaning it is much better to wait until later in the game to evolve it, but by that point in the game, Wigglytuff will have long since been useful.
2 Invest: Your Starter
It should go without saying that no matter which starter Pokémon you chose, you should be investing in it throughout the game. This is particularly the case for Charmander, since it is the "hard mode" pick of the three and isn't good against any gym leaders until Erika– although it could be argued that a Chameleon with Dig is good against Lt. Surge. There is also a distinct lack of early game Fire-types in Red & Blue, meaning Charmander is likely going to be the only one players will see until Cinnabar Island and the Pokémon Mansion.
1 Leave Alone: Rattata
Rattata has a couple of things going for it that makes it a tempting early game investment. For one, it evolves fairly early and learns some strong attacks such as Hyper Fang, but that's where the points in its favor end. Raticate quickly loses usefulness once the player's Pokémon start evolving themselves, and is practically useless from Rock Tunnel onwards. If players want a good Normal-type, it is best to wait until they can catch Snorlax or Tauros.