I have used a pure gen I team throughout the game (excluding HM slaves), their moveset and rating is as follows:
I have spent an hour or more to catch this...(5% chance on rippling water) and it has launched perish song which almost forced me to use the master ball (I caught it in the second turn with a net ball)
Despite all the difficulties at the start, it really did pay off later on in the game. Its IceBream / Surf / Psychic means that it's a beast in the cave and can easily beat Iris and the fighting dude of elite 4. It's also my HM slave in water.
It also has impressive bulk which is very helpful. I need to switch a lot in battles as I couldn't tell which poke is which by the jap name.
I would rate it as mid-high. Main drawback is lack of speed and typing.
Eevee is slightly easier to catch than Lapras and available relatively early in the game. I didn't evolve it until it learns baton pass so it is more or less useless until that point.
After evolving to Jolteon, its immense speed and SpA makes it a great sweeper. However, its movepool is quite swallow and the only "coverage" move it had was Shadow-ball. I have run TWave / Bolt / ChargeBeam / ShadowBall. It's fairly useful for any flying or water creatures and the shadowball is super effective with 2 elite 4s.
The main drawback is its frailty which gets it killed a lots of time against wild poke when i attempted to catch them.
Again I would rate it as mid-high.
-some power plant near roxie's gym
I had a Growlithe early in the game and did not evolve it until it learns outrage. This effort did not pay off though as I found it not as powerful as Lapra's IceBeam. Somehow my arcanine is lack of firepower. Even the nature is +atk, its flamewheel always fail to kill some super effective targets. And when it is using flare blitz, its survivability suffers. Extremespeed isn't that useful as well, it always fail to 2HKO somehow and lack of PP is very frustrating.
I would rate it as mid
If I was not bound to gen I poke, I wouldn't have pick it. If marowak / dugtrio is available, I wouldn't have pick it either. Nevertheless, it can learn earthquake (or magnitude in the earlier game). It is also an infinite escape rope due to Dig. My biggest complain is that why StoneEdge is not available before elite4? It is forced to use RockTomb and it has very low accuracy and power.
I have not used it at all during elite4 and champion. I would rate Sandslash as low-mid. Main drawback is lack of stone-edge, and low speed and relatively frail.
Last but not least, raticate is my big surprise in game, not because it's top percentage of rattata but it's surprisingly powerful facade.
I did not catch it until I completed champion road and need my 5th poke. I have run the set with sucker punch / quick attack / facade / filler. Facade is extremely powerful. For instance, my lv. 58 raticate can give about 80% damage to iris' haxorus, which means a 2HKO when I used suckerpunch in the second turn.
However, it is in general not advisable to use the toxic orb before those main battle otherwise a lot of investment is required on potions. No matter what, I would rate Raticate as mid.