Dynamax Omastar

Dynamax Omastar is a situationally great tank thanks to its solid bulk and set of useful resistances, cardinal of which is its double-resistance to Fire. Offensively, it is the first viable Rock Max Attacker in Pokemon GO, but it'll eventually be outclassed in the future. It falls behind the big-names in most cases, but it's not the worst investment when just starting with Max Battles.

Best Dynamax Movesets

• Water Gun and Mud Shot both have very short windows, meaning Omastar charges up the Max Meter quickly to hit its Max Moves faster. Water Gun tends to be preferable due to STAB and its higher damage.
• Rock Throw allows Omastar to be a respectable Rock attacker, but its slower duration makes it unviable defensively. Keep in mind that Omastar needs an Elite Fast TM to learn Rock Throw.
• Mud Shot or Water Gun can situationally be used offensively to turn Omastar into a Ground or Water Max Attacker, respectively, but Omastar is vastly outclassed by many other Pokemon in these roles.

In PvP, Dynamax Pokemon are identical to their basic versions, except they can't use Return.