This team uses Turtwig as the starter. There will be no version exclusive Pokémon, no trade evolutions, and no Pokémon that are exclusive to the Great Marsh. Natures and hold items are included, but they're optional. Bold denotes STAB attacks.
Torterra @ Leftovers
Overgrow; Adamant/Impish/Careful
Woodhammer/Razor Leaf
Earthquake
Crunch/Protect
Rock Slide/Leech Seed/Curse
Turtwig is, of course, gifted to you by Professor Rowan. Torterra has high HP, attack, and defense, but its other stats range from subpar to abysmal. It has just base 56 speed, so it won't be outspeeding anything. Woodhammer should be the main grass type STAB, but it has recoil and requires a Heart Scale. If you don't like the recoil, then Razor Leaf is an option. However, it is not very powerful and it inaccurate. Earthquake is incredibly powerful ground type STAB that is learned at level 32. Crunch is coverage that is learned at level 45. Bite will serve you well before you get Crunch. The last slot is up to you and how you like to play. Rock Slide is good coverage, but it has 90% accuracy and its flinch chance in nullified by Torterra's low speed. The TM for it is found in Mt. Coronet, and it requires Surf and Rock Climb. Leech Seed can be used to recover some health every turn, and it can be learned at level 33. It can be used in conjunction with Protect if you don't want to use Crunch. Curse is good for raising attack and defense. The speed drop isn't that bad due to Torterra's already low speed. Curse is learned by Turtwig at level 17.
Staraptor @ Shell Bell (Soothe Bell until max happiness)
Intimidate; Adamant/Jolly
Return
Brave Bird
Fly/Aerial Ace/Steel Wing
Close Combat
Starly can be caught immediately after obtaining Poké Balls. Staraptor has great attack and speed. Return is very powerful normal type STAB when Staraptor is at max happiness. The TM for it is found in the Lost Tower. Brave Bird and Close Combat are learned by level up at levels 49 and 34 respectively. If you are not going to have an HM slave for Fly, then it will need to go on Staraptor. Otherwise, you can use Aerial Ace (level 28) or Steel Wing (Route 209, requires Cut). Aerial Ace gets STAB and never misses. It is more powerful than Steel Wing after STAB, but Steel Wing provide coverage and a chance to raise Staraptor's defense.
Luxray @ Magnet/BlackGlasses/Metal Coat
Intimidate; Lonely/Mild/Hasty/Adamant/Jolly/Neutral Nature
Thunder Fang/Spark
Thunderbolt/Discharge
Crunch
Iron Tail
Shinx, like Starly, can be caught immediately after obtaining Poké Balls. Luxray has great attack and special attack, but its other stats are just average. Thunder Fang or Spark can be used to take advantage of Luxray's high attack stat. Both are learned by level up, at levels 42 and 18 respectively. Thunder Fang is generally better due to its flinch chance, but this is only useful against oponents that Luxray can outspeed with its base 70 speed. Either Thunderbolt or Discharge are also good options for Luxray. Discharge is learned by level up at level 56, and Thunderbolt is learned by the TM in Valley Windworks. Crunch is good coverage that is learned at level 35. Iron Tail is also good coverage, albeit innaccurate. It's found on Iron Island.
Floatzel @ Mystic Water/Nevermelt Ice
Swift Swim; Adamant/Jolly
Waterfall
Surf/Aqua Jet
Crunch
Ice Fang/Ice Beam
Buizel is found at Valley Windworks, which you can reach after obtaining the first badge. Floatzel has great speed and attack, but it has very bad defensive stats. In short, its a glass cannon. Waterfall and Surf are STAB attacks that double as HM moves. You could put Surf on a Bibarel and run Aqua Jet (level 21) on Floatzel, but Surf is generally better (even with an Adamant nature). Crunch (level 26) and Ice Fang (move relearner) are for coverage. Ice Fang is generally better because of its flinch chance, but Ice Beam is an option for those who can't/don't want to obtain a Heart Scale. Either way, Floatzel needs an Ice type move to KO Cynthia's Garchomp.
Rapidash @ Charcoal
Flash Fire; Adamant/Jolly
Flare Blitz
Flamethrower/Fire Blast
Megahorn
Poison Jab/Bounce/Stomp
Ponyta can be found fairly early on. The earliest you can encounter one is on Route 206. Rapidash has great speed and attack, like Floatzel. However, Rapidash's defenses are much better than that of Floatzel. Flare Blitz is a powerful fire type STAB attack that is learned at level 58. Unfortunately, it is the only fire type move that takes advantage of Rapidash's high attack stat. Flamethrower and Fire Blast will be used as the main fire type attack until level 58. Fire Blast is learned at level 38 and Flamethrower is found in Fuego Ironworks. Megahorn and Poison Jab are learned via the move relearner and require a Heart Scale. Alternatively, you could use the TM for Poison Jab, which is found on Route 212. However, this will prevent you from using it on the next Pokémon on the team. Bounce is an alternative to Poison Jab, and it is learned at level 49. You may also choose to use Stomp, which is learned at level 19 (shortly after you catch Ponyta).
Weavile @ Nevermelt Ice/Blackglasses
Pressure; Adamant/Jolly
Night Slash
Avalanche/Ice Shard
X-Scissor
Aerial Ace/Poison Jab/Metal Claw
Sneasel is found rather late in the game on Route 216. It is the final member of the team. To evolve Sneasel, you will need to locate the Razor Claw on Route 224 (or Victory Road). Weavile has very good stats, but it has a poor movepool. Its only options for a physical ice type attack are Avalanche and Ice Shard. Avalanche has decreased priority, nullifying Weavile's high speed stat. Ice Shard has less power, but it has priority. However, Ice Shard is only learned by Sneasel at level 49. The TM for Avalanche is earned in the Snowpoint City Gym. Night Slash is great dark type STAB because of its base power and high critical hit ratio. Weavile learns this move at level 35. X-Scissor is learned by the TM found on Route 221. Aerial Ace is learned by the TM found on Route 213. Poison Jab is learned by the TM found on Route 212. Metal Claw is learned by Weavile at level 42.