Magmortar

Magmortar does not get Overheat, the superior Fire-type charge move. As a result, it loses to many other Fire-types. Magmortar is definitely serviceable, just not outstanding.

Best PvE Offensive Moveset

• Fire Spin is the move with STAB and suitable for Magmortar as a Fire specialist.
• Fire Punch and Fire Blast are very close, with the former being more dodge friendly.
• Brick Break and Psychic are just not appropriate on Magmortar. TM those.
• Thunderbolt* and Scorching Sands lack STAB and don't have any use on offense. 

Best PvE Defensive Moveset

• Fire Spin is much more damaging than Karate Chop on defense.
• Brick Break is hard to dodge and is not resisted by Magmortar’s common counters.
• Fire Punch is stronger than Brick Break and also has 3 bars.
• Psychic and Scorching Sands have relevant defensive coverage and 2 bars.
• Fire Blast is not likely to get used at all before Magmortar faints. 
• Thunderbolt* might be useful to hurt Water-type attackers, but that's about it. 

Karate Chop is typically preferred over Fire Spin for its coverage and slightly better overall parameters. However, Fire Spin can still be ran for a more reliable Fire-type damage.

Fire Punch is Magmortar's primary charge move, as it benefits from STAB while also costing low amounts of energy. Brick Break is a very weak move, but costs the least amount of energy, while also providing Fighting-type coverage. Thunderbolt provides coverage against Magmortar's weakness to Water Pokemon, though it's moderate energy cost makes it more difficult to access before fainting. Scorching Sands can be useful for counter-coverage against Rock Types and can debuff Attack, but it can also be hard to justify its use over other options. Psychic could also be chosen for its typing coverage depending on the format, though it is slower to access as well.