0131 Lapras

Ice Shard is more useful than Frost Breath or Water Gun. Ice has better coverage than Water, and Ice Shard's energy generation makes it more important than Frost Breath's damage. Bummer that it's Legacy, though.

Surf is 100% mandatory on Lapras, providing very solid and consistent Water-type damage. Your secondary move of choice will depend on the format. Skull Bash is at its strongest in Great League and Ultra Premier, where you're likely to run into Pokémon that would resist a Water/Ice moveset (e.g. Azumarill, other Lapras). Ice Beam mostly sees play in the Open Ultra format, letting you really need that Ice-type nuke to hit Giratina.

Dragon Pulse was made a legacy move starting February 21, 2017, and now available through Elite TMs. Demanding 60 energy to use, it's slightly slower than Ice Beam, but does offer the perk of neutral damage against opponents that would resist Water or Ice-type attacks (AKA other Water-types). It's also handy in the Lapras mirror match, and easier to get to than Skull Bash, but does not offer a DEF self-buff. 

Lapras is known as a bulky Water/Ice-type with a powerful moveset, some of which are legacy movesets. The limited access to rare legacy movesets generally makes Lapras highly desired in trades, as having a certain moveset advantage over opponents could potentially be enough to secure wins. While Lapras is a solid pick for PvP and is quite popular, it does have to compete with many other Water-type picks like Azumarill who tend to perform better overall. Weaknesses to four common typings can also be a problem against other meta picks. 

Lapras does incredibly well in Ultra League, with a high overall win rate. Lapras' primary role is countering Giratina, though its bulk gives it strong neutral matchups against a lot of the format. In Ultra Premier, it's a meta-defining mon in its own right, handling common threats like Dragonite and Swampert. No matter the format, though, beware Grass-, Fighting-, and Steel-types.

With a max CP of 2641, Lapras lacks sufficient stats to perform well in Master League. Lapras does narrowly beat Giratina Origin Forme, but it loses to the Altered Forme and Dialga. So while Lapras is an overall good Pokemon with terrific moves and bulk, it's mostly limited to the Great and Ultra Leagues.