Unova is the fifth region and generation of Pokémon, and it was accompanied by the Pokémon Black and White anime and video games. Brock finally departed from Ash's team and returned home to Kanto at this point, and Dawn stayed in Sinnoh as well. Ash was joined by Iris and Cilan, both of whom are gym leaders in the games.
Like previous regions, Ash caught a handful of Pokémon in Unova, though not as many as he did back in Kanto. Many of these Pokémon proved to be valuable partners, but some were stronger than others.
9 Palpitoad
Palpitoad is one of those rare Pokémon that Ash catches already-evolved. Palpitoad led a group of Tympole, and Ash and his Oshawott accidentally invaded their territory while looking for medicinal herbs to cure a poisoning. Oshawott battled Palpitoad, and Ash caught Palpitoad upon Oshawott's victory. It won an early match against a Stoutland and proved victorious over Gym Leader Elesa's Zebstrika. However, Palpitoad lost against her Emogla afterward. Palpitoad faced another Zebstrika at the Vertress Conference, but this turned into a tie.
8 Scraggy
Ash received Scraggy as an egg. When it first hatched, it proved gutsy and aggressive, but it was put in its place by Pikachu. Scraggy formed a rivalry with Iris' Axew, and the two battled to a draw several times--though Axew finally won on the fourth battle. It helped Team Rocket's Meowth rescue Iris' Axew from a Scrafty then went onto help that Scrafty take back its home from a Mandibuzz. Scraggy later showed its fighting prowess by winning two matches at the Clubplosion event, beating a Simisage then a Simipour.
7 Oshawott
Ash received Oshawott from Professor Juniper, and it earned Ash an early victory against Cilan's Pansage despite the type disadvantage. Oshawott struggled a lot after this victory though, even losing its Scalchop for a time. He also had trouble controlling his Aqua Jet, which Ash helped Oshawott master with some help from fellow trainer Trip.
Oshawott's Aqua Jet was pivotal in Ash catching Palpitoad. Oshawott later won out against Clay's Krokorok but was taken down by the Gym Leader's Palpitoad. He later beat out a Dewott, the evolved form of Oshawott, for the love of another Oshawott. However, this didn't win that Oshawott's heart.
6 Leavanny
Ash encountered this Pokémon as a defiant Sewaddle that came to trust Ash after he helped save it from a Woobat and a Patrat. Sewaddle served Ash well during his battle against the Bug-Type Gym Leader, Burgh. He took down his Dwebble and, after evolving into Swadloon, Whirlipede. It was eventually beaten by Burgh's own Leavanny. Upon evolving during a battle with Iris' Emolga, Leavanny turned into a nurturing parent-like Pokémon. It constantly made leaf hoods and clothing for Ash, his friends, and their Pokémon.
5 Boldore
Ash caught Boldore back when it was a Roggenrolla, and it defeated Ash's Oshawott and Tepig in battle. Team Rocket arrived and stirred up trouble in the cave Roggenrolla lived in, and the Roggenrolla helped Ash's team defeat Team Rocket. Eventually, Roggenrolla learned to trust Ash and became part of his team. He achieved some impressive feats while under Ash's care, particularly against Team Rocket. It made use of powerful moves like Stone Edge and Flash Cannon to foil many of their plans and became known for its ability to endure attacks. It was Ash's choice against Gym Leader Clay and his Excadrill. Roggenrolla endured Excadrill's powerful attacks until it evolved into Boldore and turned the tide of battle. This was Boldore's peak moment, though it did score a victory for Ash against a Reuniclus in the Pokémon World Tournament Junior Cup.
4 Snivy
Ash had to work hard to win Snivy's respect, but he finally succeeded and caught Snivy with the help of Pidove. Snivy scored some early victories against Servine and a Pawniard. Snivy quickly formed a rivalry with Iris' Emolga, and the two held it for the entire Unova journey. Snivy battled bravely against the Legendary Pokémon Tornadus and Thundurus but was no match for their power. She later took down Gym Leader Clay's Palpitoad, then she was taken out next by Clay's Excadrill. Snivy showed great bravery and compassion when it took a powerful combination of Fire-Type attacks from Pokémon Trainer Shamus that was intended for Tepig. Shamus was Tepig's previous trainer, and he treated Tepig poorly. This act of sacrifice motivated Tepig to evolve into Pignite, and he won the battle.
3 Unfezant
Ash met Unfezant as a Pidove, and she was the first Pokémon he caught in Unova. Pidove was a key part in Ash catching his Snivy, and she evolved into Tranquil while helping to clear a Venipede infestation in Castelia City. During Ash's challenge against the Flying-Type Gym Leader, Skyla, Tranquill was able to take down Skyla's Swoobat and evolved into Unfezant while battling Swanna--winning that match.
Unfezant also helped stop the Legendary Pokémon Thundurus, Tornadus, and Landorus when they were being controlled by Team Rocket's Giovanni, and it helped save Meloetta.
2 Pignite
Like Chimchar in the Sinnoh region, Pignite, then a Tepig, was abandoned by a cruel prior trainer, this one named Shamus. Ash found and took Tepig in. He immediately scored a victory in a Gym challenge against Chili's Pansear. After a couple of defeats, it took down a Lenora's Herdier. Tepig and Snivy later challenged Shamus, and, after Snivy took a heavy super-effective attack for Tepig, Tepig evolved into Pignite and beat Shamus' Emboar and Heatmor. Pignite went on to beat Gym Leader Roxie's Scolipede, though he lost to her Garbodor right afterward. At the Vertress Conference, Pignite went on to take down a powerful Hydreigon and a Ferrothorn. On the Decolore Islands, Pignite had a winning streak against a pair of pirate Pokémon, Croconaw and Azumarill, and won a sumo competition against a Watchog, a Slaking, a Rhyperior, and a Golurk.
1 Krookodile
Krookodile first appeared as a Sandile that decided Pikachu was its rival. It wore sunglasses all the time, and it battled Pikachu on a few occasions, eventually evolving into a Krokorok during one such showdown. Ash caught Krokorok after this. Krokorok's initial battles alongside Ash were unsuccessful, losing to Gym Leader Skyla's Swoobat, failing to follow through with Gym Leader Brycen and his Beartic's training techniques, and losing to a poacher's Jellicent while trying to protect Volcarona. However, Krokorok ended up beating Beartic in the Gym battle against Brycen. It then evolved into Krookodile during a battle with Iris' Dragonite during the Pokémon World Tournament Junior Cup and won with a powerful Stone Edge. Krookodile even battled the Legendary Pokémon Meloetta, it took down a Liepard and a Sawk during the Vertress Conference. Krookodile was also key in helping Ash and Looker arrest the dastardly Team Plasma.