Bulbasaur as starter: Venusaur, Dugtrio, Jolteon, Lapras, Snorlax, Fearow
Venusaur: Razor Leaf/Frenzy Plant, Sleep Powder, Cut/Strength/Sludge Bomb, Earthquake
Your starter evolution if you chose Bulbasaur.
Razor Leaf provides a STAB attack with a lot more PP than Giga Drain. Frenzy Plant is actually a good move for an ingame run, so it can be used as a good finishing blow to the foe. Unfortunately SLudge Bomb is only available as a TM later in the game, after the Elite 4, and as such is not as much a viable ingame move. You can replace Cut for it when you do get it though. Strength can also be used as an HM over Cut, if your Snorlax is wanting Thunderbolt or Thunder this is a good idea. Sleep Powder can give Venusaur the chance to win some otherwise tough battles and also makes it easier to catch wild Pokemon. Cut is an HM. Earthquake is high powered and provides coverage.
Dugtrio: Earthquake, Aerial Ace, Rock Smash/Cut, Rock Slide
You can catch Dugtrio at Level 31 in Diglett's Cave, which at that point in the game might be a higher level than your starter Pokemon. If not, then it will still likely be a higher level than the rest of your team. With high Speed and solid Attack, Dugtrio is a solid Pokemon for ingame. Arena Trap even makes wild encounters more likely, which can be helpful for training or finding a specific Pokemon. Earthquake is very high powered STAB with great coverage. Aerial Ace is coverage, as is Rock Slide. Rock Smash is an HM move, although if Venusaur is using Sludge Bomb or Strength you might want to use Cut.
Jolteon: Thunder/Thunderbolt, Pin Missile, Double Kick, Rain Dance/Flash
A powerful Electric type, it has excellent sweeping stats and so can do huge damage. Volt Absorb is also a solid ability, as immunities are even better ingame than in competitive. It might not have much of a movepool but it still proves itself a helpful member of the team. Thunderbolt is unfortunately a TM, so if you want Lapras to have it, Thunder is the way to go. Thunder is also the way to go if using Rain Dance. Pin Missile helps out against the Abra family. Double Kick against Normal types such as Snorlax. Rain Dance gives Thunder 100% accuracy, strengthens Lapras's Water type attacks, and removes Venusaur's Fire weakness. Flash however can get rid of your foe's accuracy, making wild Pokemon and stronger foes do nothing to you, as well as provides you with an HM. However, Flash is not one of those needed HMs, as some people can navigate through the caves without its use. If you have trouble with this though, Flash is the way to go.
Lapras: Surf, Waterfall, Ice Beam, Thunderbolt/Thunder
Your Water HM Slave. It is used over other Water types mainly due to its STAB Ice attacks. Lapras proves incredibly useful against the Elite 4. Surf is considered by many to be the most important of the HM moves and it also is an awesome attack. Waterfall can back up a Surf that is out of PP, but more importantly it is an HM. Ice Beam is a great STAB move. Thunderbolt is coverage, although Thunder can be used if running Rain Dance on Jolteon.
Snorlax: Return, Shadow Ball, Flamethrower, Strength/Thunderbolt/Thunder/Focus Punch
Snorlax is an excellent Pokemon, having high stats, two useful abilities, and a good movepool. It also comes at a decently high level, not Dugtrio high, but good enough for that point in the game. Return hits hard with STAB. Shadow Ball gets coverage as well as hits any Gengars that Agatha uses against you. Flamethrower provides coverage also. The last move has mutliple solid options. Strength is an HM that backs up the PP of Return. However, you can use Thunderbolt or Thunder (depending which you used on Lapras) to provide type coverage. Focus Punch is the final choice, and it can be useful as an incredibly high powered attack when your opponent uses a status move. However, due to the unpredictable nature of the AI, this might not be your best option.
Fearow: Drill Peck, Fly, Steel Wing, Hyper Beam
Your Flying type. You get it early on in the game, meaning you can start training it right away. Fly is a favored HM because it lets you get from place to place with ease. Drill Peck is a much more useful move for battling though. Steel Wing provides type coverage. STAB Hyper Beam is a powerful finishing move, and is actually good for ingame use.