Vaporeon - Water Pulse, Blizzard, Bite, Ice Beam (Item: Leftovers)
Water Pulse's TM is obtained after beating Area 2 of Mt. Battle. It's the strongest STAB move Vaporeon gets in XD. Blizzard is obtained rather early from Realgam Tower's shop. It is quite strong and hits both opponents, having various uses. Ice Beam is obtained shortly after Blizzard in the back of Phenac Stadium. It is a bit stronger than a STAB Water Pulse, and hits Grass-types that resist Water. Bite is super effective on Ghost- and Psychic-types, and it's the best thing that's there for this Pokemon. Eevee knows it from the start. Leftovers are there because of Vaporeon's high bulk. They are found on the S.S. Libra where Bonsly was standing, as well as held by Ardos' Shadow Snorlax.
Ursaring - Return, Earthquake, Lick, Metal Claw (Item: Soothe Bell)
Return is very strong when at full power, and the Soothe Bell helps boost its power. Earthquake hits both opponents, but Protect may be necessary for some of them. I tried to keep it simple with these last two moves, so you can have them right off the bat and won't have to do so much Mt. Battle to get Aerial Ace or Rock Tomb. Lick can do chip damage to Shadow Pokemon and paralyze them, and Metal Claw gives coverage on Ice-types.
Houndoom - Flamethrower, Crunch, Charm, Protect (Item: Charcoal)
Flamethrower's TM is found on the S.S. Libra, and is its best STAB. Crunch won't be learned unless you're doing the aftergame, since it's learned at level 59. Protect can guard from your own Earthquake or an opponent's attack. Its TM is bought from Realgam Tower's shop. Finally, Charm is a filler with a slight use. It's a special move that Houndour and Houndoom don't normally get. However, it has a rather low Defense stat and there are some incredibly strong physical attackers later in the game such as the Shadow Salamence, Tauros, and Rhydon that you meet in the last two battles of the game. It's known as soon as Houndour is purified. Charcoal is held by the Numel that is found in the Shadow Pokemon Lab.
Shiftry - Giga Drain, Shadow Ball, Brick Break, Fake Out/Self Destruct (Item: Spell Tag or Miracle Seed)
Shiftry is great early on because Seedot evolves with one level, and the Leaf Stone is found at the end of the Shadow Pokemon Lab, meaning you can get Shiftry very early. Giga Drain is known from the start, and though it's lacking in PP, PP Ups are common in this game and can be easily used on this move. Shadow Ball is coverage and allows you to evolve without waiting to learn Faint Attack, as Ghost and Dark are almost the same offensively. Its TM is from the locked chest in the Snagem Hideout. Brick Break is, of course, for coverage, and its TM is obtained after beating the Pyrite Colosseum for the first time. Finally, Fake Out is for some free damage against your opponent, learned by Nuzleaf at level 19. Also, Self Destruct is an option for taking out strong opponents easily, learned via the move tutor in Agate. It is only available later in the game, though. The Spell Tag is on Shadow Banette on Citadark Isle, and the Miracle Seed is on Shadow Tangela in the Cipher Key Lair.
Dusclops - Shadow Ball, Helping Hand, Confuse Ray, Seismic Toss (Item: Leftovers)
Shadow Ball is STAB, but it might not do outstanding damage. It's known from the start. Helping Hand is also known from the start, and it's just to support the attacks of your teammates when you need to focus attack on one enemy. Confuse Ray, also known from the start, is great when you want to decrease your foe's damage output. Finally, Seismic Toss does damage based on your level instead of Attack stat, so it's great for a defensive Pokemon. It does consistent damage to the opponent and can be taught with Agate's move tutor as soon as Duskull evolves. Leftovers are near Bonsly on the S.S. Libra and held by Shadow Snorlax.
Gardevoir - Psychic, Thunderbolt, Thunder Wave, Reflect (Item: Twisted Spoon)
Gardevoir is both an attacker and a supporter at the same time. I feel like it takes the place of both Espeon and Jolteon, except trading their speed for special defense. Psychic is, of course, Gardevoir's most reliable STAB move. Kirlia learns it at level 26. Thunderbolt is used for coverage against Flying, Water, Dark, and other types. Its TM is in the Cipher Key Lair. Thunder Wave, learned using the move tutor in Agate Village, is great for lowering the speed and threat of fast Pokemon, and more notably making Shadow Pokemon easier to catch. Finally, Reflect is used to guard from attacks on Gardevoir's weaker defensive stat, as well as protecting the rest of the team. It is known by Ralts from the start. Unfortunately, Colosseum and XD use RSE's level-up moves instead of FRLG's, meaning Kirlia can't use the move reminder to get Magical Leaf. Ralts comes holding a Twisted Spoon.