Vaporeon

Who would have known! The era of Vaporeon has ended in such a way that Vaporeon is surpassed by the one Pokemon who had lived in its shadow forever - Gyarados. But if Gyarados turned the final page, then Kyogre closed the book, as Vaporeon is now entirely outclassed on every metric when the king of the seas made its grand debut. That said, Vaporeon is still a decent budget option, as Eevee is still very common in many areas across the globe.

On defense, Vaporeon’s centrality has also diminished by a large margin. Water Gun was never a good fast move on defense, and Vaporeon’s mono-Water movesets are predictable and easily countered. Most serious players today are well equipped with powerful Electric-type legendaries and Grass-type attackers. Vaporeon is still one of the bulkiest defenders, but that’s about it.

Best PvE Offensive Moveset

• Water Gun is Vaporeon's best Fast Move... Water Gun is Vaporeon's only Fast Move.
• Hydro Pump sports the highest DPS but risks losing DPS is Vaporeon drops with energy left over. Liquidation is 2-bar and easier to utilize, but a little lower in DPS. Aqua Tail is 3-bars but has DPS a bit lower than Liquidation. Honestly, any of the three is fine.
• Water Pulse is the loser, having low damage and cooldown on par with Hydro Pump.
• Scald* is a direct upgrade to Water Pulse, but is still behind other options and is not worth the Elite TM. 
• Last Resort* has little to no offensive viability.

Best PvE Defensive Moveset

• Aqua Tail has 3 bars and an early damage window, harassing attackers and punishing charge moves with long cooldown, while Last Resort* is Vaporeon's only non-Water Charged Move.
• Liquidation is fine, but behind Aqua Tail on defense.
• While Hydro Pump is a 1-bar charge move, its high damage and early damage window will devastate inattentive attackers.
• Scald* is an acceptable option for defense as a 2-bar move, but its limited accessibility is not worth the Elite TM. 
• Water Pulse's late damage windows makes it the least desirable option.

Water Gun is a good move and Vaporeon's only Quick-Move option, not to mention it's only option... as we've already mentioned.

Aqua Tail gives Vaporeon an quick-charging, decently powerful Charge Move to spam frequently. Scald received a buff following the GBL Season 9 update and is now more viable as a secondary move. Since its chance to debuff the opponent is only 30%, this is not a consistently reliable strategy, though Scald does deal more Damage Per Energy (DPE) if unshielded. Hydro Pump demands a huge amount of energy to use, making it less desirable. 

This set foregoes the power of Hydro Pump or debuff potential of Scald to give Vaporeon an option in particularly bad matchups via the aptly named Last Resort. It can provide better power VS Grass, Dragon, and fellow Water types, though against Grass types in particular it's probably best to try to get Vaporeon out ASAP.