From all I've seen, heard, and researched, this game kind of plays itself in the sense that it is an easy and casual game, so I don't think over-the-top effort or optimal natures are required for an in-game team to work. :P
Bolded moves can only be obtained from a prior Evolution.
Locations of the Pokémon's base form are next to it's name.
Here are the teams I suggest:
Lets Go Pikachu:
Optimal Nature: Naive / Adamant
- Thunderbolt (21) / Zippy Zap (Celadon/Fuchsia Tutor)
- Quick Attack (6) / Floaty Fall (Celadon/Fuchsia Tutor)
- Double Kick (9) / Splishy Splash (Celadon/Fuchsia Tutor)
- Thunder Wave (15)
Splishy Splash covers the Pokémon Poster Child's Ground weakness, while Double Kick is useful when you get to Brock. Thunder Wave can be useful whenever Partnerchu isn't strong enough to Take out the opponent on it's own, and helps slower team members take out threats. Zippy Zap is probably the best STAB priority move in the game, and Floaty Fall abuses Pikachu's boosted Base Speed to make use of a 30% flinch chance and a reliable typing for hitting Grass types.
Venasaur (Chain encounter for Bulbasaur in Vidiran Forest)
Optimal Nature: Modest / Calm
- Leech Seed (9)
- Sleep Powder (14)
- Tackle (1) / Sludge Bomb (TM 52)
- Vine Whip (5) / Petal Dance (Evolving into Venasaur)
Bulbasaur Helps meet the requirement of having a Grass type before Brocks Gym. Venasaur is a great tanky Pokémon with a High Special Attack. It's versatility is backed up with Leech Seed and Sleep Powder to deny the opponent turns and sap their HP. Sludge Bomb hits hard, and Petal Dance is better in-game, as it usually KO's 1 or 2 Pokémon before Venasaur becomes confused. Vine whip is very useful against Brock and Misty, and will do eccentrically against Bruno in the Elite Four. Venasaur becomes even bulkier if you Mega Evolve it.
Fearow (Route 3)
Optimal Nature: Adamant / Jolly
- Drill Peck (32) / Fly (TM 14)
- Drill Run (TM 58/Reminder)
- Roost (24, TM 50) / U-Turn (TM 18)
- Fury Attack (16) / Quick Attack (Reminder) / Toxic (TM 27)
Fearow is fast, gets Fly (if you need it) and Has a great Attack Stat. It gets a variety of moves useful to abusing it's Speed, such as U-Turn, Toxic, and Fly. Fearow can also switch in to Ground Type attacks most of the team is weak to.
Dugtrio (Diglett's Cave, Lavender Town)
Optimal Nature: Jolly / Adamant
- Dig (18, TM 10) / Earthquake (29, TM 41)
- Sucker Punch (25)
- Rock Slide (TM 22) / Toxic (TM 27)
- Slash (21) / Fissure (33, 45)
Dugtrio's Stats are similar to Partner Pikachu's, and Dugtrio is a fine option for Ground STAB. Dugtrio has many ways of abusing it's high Speed Stat, such as Rock Slide, Toxic, and Dig.
Arcanine (Route 5, 6, 7, 8)
Optimal Nature: Hasty / Lonely (Jolly / Adamant may work)
- Flamethrower (17, TM 37) / Heat Wave (24)
- Crunch (28)
- Play Rough (35, TM 44)
- Outrage (31, TM 39) / Will-O-Wisp (TM 11)
Lets abuse the Pikachu version-exclusives Growlithe and Arcanine. It gets average defenses, decent Speed and HP, and Above average Attack and Special Attack. Because of this, it is a great addition to the team as it can attack both Physically and Specially, as well as covering Grass, Dark, Ice, Bug, and Steel type Pokemon better than most of the Team. You could replace Arcanine for Charizard, but that may create conflicting Mega Evolutions, as Charizard can only use Physical Fire type moves effectively as Mega Charizard X. It should also be noted that Growlithe/Arcanine Level Up faster than Charizard.
Tentacruel (Route 4, 10, Seafom Islands, etc.)
Optimal Nature: Calm / Timid / Bold
- Surf (27, TM 47) / Scald (TM 29)
- Barrier (31)
- Wrap (22) / Ice Beam (55)
- Sludge Bomb (TM 52)
Tentacruel is pretty bulky and fast, and deals a fair bit of special damage. It can Trap and Stall with Wrap and Barrier, and hits decently with Sludge Wave and Surf. Scald may Burn and Cripple Physical attackers. One Barrier use brings it's Defense phenomenally high, and considering the lack of status clearing moves, makes it quite dangerous for the NPCs to handle.
Lets Go Eevee:
Optimal Nature: Bold / Timid / Modest
- Quick Attack (6) / Glitzy Glow (Celadon/Fuchsia Tutor)
- Double Kick (10) / Bouncy Bubble (Celadon/Fuchsia Tutor)
- Bite (17) / Baddy Bad (Celadon/Fuchsia Tutor)
- Swift (21) / Sparkly Swirl (Celadon/Fuchsia Tutor)
Eevee has average stats overall, but gets amazing Special Moves such as Bouncy Bubble that heals, Glitzy Glow and Baddy Bad that set Light Screen and Reflect respectively, and Sparkly Swirl; the only move that heals party status in the whole game. Eevee is rated the better of the two starters in Lets Go, mainly because the variety of it's exclusive moves is greater than Partner Pikachu. It can be said, however, that Eevee is more defensive, while Pikachu is more offensive.
Victreebel (Route 1, 2, 12, Vidiran Forest, etc.)
Optimal Nature: Impish / Relaxed
- Razor Leaf (22) / Power Whip (54)
- Growth (4) / Swords Dance (Reminder)
- Poison Jab (26, TM 26)
- Leech Life (Reminder) / Sucker Punch (Reminder)
Victreebel is a Lets Go Eevee exclusive. It doesn't have great defenses or Speed, but it'll Level Up fast, has great Attack, and is definitely useful for an In-Game Team. If you wish, you can use the Bulbasaur line instead.
Charizard (Chain encounter for Charmander in Rock Tunnel)
Optimal Nature: Timid / Calm / Naive
- Flamethrower (36, TM 37) / Heat Wave (71, Reminder)
- Dragon Rage (22) / Dragon Pulse (TM 34)
- Air Slash (Reminder) / Roost (TM 19)
- Roost (TM 19) / Dragon Claw (Reminder, TM 2)
Chaizard has a great set of moves and can pull it's own in an in-game team. Use Dragon Rage early on, but loose it when you get Dragon Pulse. Roost boosts Charizard's survive-ability, while Air Slash is for STAB and a flinch chance, and Dragon Claw is excellent coverage to specially defensive Pokémon that may wall Charizard. This set works for Mega Charizard X and Mega Charizard Y, so if you change your mind, you can.
Nidoqueen (Route 9, 10, 22, 23)
Optimal Nature: Adamant / Impish / Careful
- Double Kick (9) / Superpower (55)
- Crunch (24) / Dig (TM 10) / Earthquake (TM 41)
- Toxic (18, TM 6)
- Poison Jab (TM 26)
I chose Nidoqueen over Nidoking simply because Nidoqueen was more bulkier than the extra Attack Nidoking Had. Nidoqueen can be hard to break through and gets many powerful physical moves to help along the way. Double Kick is for beating Brock easily.
Poliwrath (Route 22, 23, 25, Cerulean Cave)
Optimal Nature: Impish / Adamant
- Waterfall (TM 25)
- Low Kick (26) / Super Power (57, TM 49) / Submission (Reminder)
- Earthquake (TM 41)
- Bulk Up (TM 30, Reminder)
Poliwrath is one of few Pokémon that get stat boosting moves. It can take out most threats with or without the boost, and it has great bulk to help it survive hits.
Electrode (Power Plant)
Optimal Nature: Modest / Bold
- Light Screen (3)
- Thunderbolt (31, TM 36)
- Thunder Wave (TM 16) / Reflect (TM 9)
- Foul Play (TM 21) / Toxic (TM 27)
If you haven't already been talked to death about abusing Speed, I am genuinely surprised. Electrode has one of the highest base Speeds in the game, so it doesn't Really need a +Speed nature. It cane be a screen setter, status inflict-er, a counter to Powerful physical attackers, and a decent special . And it'll be the fastest like no one ever was while doing it. :P
Well, that's my take on it.
Hope I helped!