Turtwig - A, a beginner Pokemon so it is obviously easy to obtain. However, it is slow and doesn't learn Earthquake until evolving much later. Once it finishes evolving, you can easily have a moveset of EQ / Wood Hammer / Curse / Stone Edge by the time you reach the Elite 4. Ice-type attacks are rather scarce in this game but Turtwig is generally useless during the 7th gym (Team Galactic focusing on Flying-types such as the Zubat line and Murkrow, while Candice's gym specializes in Ice).
Piplup - B, There are plenty of other good Water-types such as Gyarados and Buizel who will come along later. Piplup is useful for the first gym but has trouble with a lot of the other ones. Piplup doesn't learn many useful attacks until much later and even then, they are moves like Bubblebeam. With Magikarp and Buizel nearly effortless to catch, it is almost dumb to pick this over Turtwig or Chimchar.
Cranidos - C, it takes a lot of luck to actually obtain Cranidos' fossil, even if you can get it before the second gym. Cranidos takes an extremely long time to level up while young, but eventually it grows extremely fast. It can make a great glass cannon attacker ingame and can usually defeat the 1st and 3rd of the Elite 4, depending on what attacks you give it.
Shieldon - D, Shieldon takes the same amount of work to find as Cranidos but lacks Cranidos's power. Trying to level it up is a pain and it is weak to a lot of common types (with Machop and Geodude everywhere).
Machop - B, can get it very early (before the 1st gym) and a strong Fighting-type can really help against all of the Normal- and Rock-types that you are sure to find. Not too helpful late in the game and it requires trading to evolve, but it can help a great deal early game. Also makes a good HM Slave with Rock Smash and Strength.
Psyduck - C, easy to obtain but outclassed by other Water-types, Gyarados and Buizel.
Combee - D, Very difficult to obtain (slather Honey on a tree and wait a day), has almost no attacks, and only female versions evolve. Vespiquen is still rather weak compared to other Bug- and Flying-types.
Pachirisu - C, Luxray is much more powerful and easier to obtain. Luxray's Intimidate can also win a few matches by itself, so if you are looking for an Elc-type, Luxray is the way to go. I changed my score from D to C, because Pachi gets Pickup.
Buizel - A, is very easy to obtain, has great Attacking and Speed stats once evolved, can learn Waterfall, Crunch, Ice Fang, etc. Can be a great help against the 2nd, 3rd, and 4th of the E4.
Shellos - B, has a decent typing but doesn't match the power of Buizel or Gyarados.
Heracross - B, it is extremely rare to find on the Honey Trees and it takes an entire day to wait. It learns very good attacks though and has an extremely high Attack score.
Drifloon - C, if you don't start on the right day, you will have to wait up to an entire week to even see Drifloon. Gastly outclasses it in every category but tanking, but the game is focused on Speed and attacking, not playing out stall wars.
Gastly - A or S, 3 extremely useful immunities and a very high Special Attack and Speed, even if you don't get around to completely evolving it. Evolution of Haunter to Gengar can happen anytime by simply trading. Gastly can know attacks such as Sludge Bomb, Shadow Ball, and Thunderbolt.
Barboach - C, outclassed by other Water-types, does learn Earthquake though.
Chatot - D, comes far too late, has a bland typing, and is outclassed by the ever present Starly.
Gible - B, very good typing, although Ice weak. Gible's final evolution, Garchomp, is extremely powerful and fast and if used correctly, could sweep most of the Elite 4. A final moveset of Stone Edge / Earthquake / Fire Blast / Dragon Claw is very easy to obtain. Biggest drawback is waiting for Exp.
Munchlax - C, very hard to obtain and isn't as powerful as other Normal-types such as Starly.
Giratina - B, it comes awfully late but it is mandatory to fight and can help a lot with the Elite 4, especially with its high HP and attacks such as Ominous Wind which can boost all stats.