Mega Rotom-N
Typing: Electric/Ghost ----> Electric/Ghost
Abilities: Levitate ----> Download
BST: 50/50/77/95/77/91 ----> 50/60/77/135/82/136
New Moves: Wild Charge (all formes), Dazzling Gleam, Flash Cannon
Since Rotom-N has mostly lower stat then the other Rotom formes, It's hard to give him a specific niche. However, with Download, Mega Rotom-N can nab a Sp.Atk boost from the ability and start firing off attacks. Timid ensures that Mega Rotom-N outspeeds many opponents, and Dazzling Gleam provides coverage against Dark types. Mega Rotom-N is still incredibly frail, and can die to relatively weak attacks.
Mega Rotom-H
Typing: Electric/Fire ----> Electric/Fire
Abilities: Levitate ----> Flash Fire
BST: 50/65/107/105/107/86 ----> 50/88/107/145/110/120
New Moves: Wild Charge (all formes), Heat Wave, Lava Plume
While Mega Rotom-H now got the 4x Ground weakness, which is basically on everything, its decent speed and a 145 Sp.Atk can remedy that. Heat Wave provides a more reliable STAB which works well with Flash Fire, as the Sp.Atk drop from Overheat might not be favorable. Lava Plume has a high chance of causing burn, which can prolong Mega Rotom-H's time before fainting.
Mega Rotom-W
Typing: Electric/Water ----> Electric/Water
Abilities: Levitate ----> Water Absorb
BST: 50/65/107/105/107/86 ----> 50/70/142/110/152/96
New Moves: Wild Charge (all formes), Surf, Soak
Mega Rotom-W is, of course, going to be incredibly bulky. With a Calm nature and 252 HP / 252 Sp. Def spread, it can tank a ridiculous amount of hits. The lack of recovery moves is a pain, but Water Absorb helps in that regard. Surf provides a weaker but more reliable STAB, and Soak allows Mega Rotom-W to hit opponents that resist both its STAB, and forces switches at the same time.
Mega Rotom-F
Typing: Electric/Ice ----> Electric/Ice
Abilities: Levitate ----> Snow Warning
BST: 50/65/107/105/107/86 ----> 50/80/120/125/123/122
New Moves: Wild Charge (all formes), Freeze-Dry, Icy Wind
Mega Rotom-W might not have the best Sp.Atk or defenses, but it can still do work with STAB Blizzard and Snow Warning. Freeze-Dry provides coverage against Water-Ground/Dragon/Electric opponents, and Icy Wind can hit switch-ins for chip damage, provide an opportunity for another hit, and to force switches.
Mega Rotom-S
Typing: Electric/Flying ----> Electric/Flying
Abilities: Levitate ----> Clear Sky (Cancels ongoing weather)
BST: 50/65/107/105/107/86 ----> 50/71/112/130/112/145
New Moves: Wild Charge (all formes), Tailwind, Defog
Mega Rotom-S provides general support for the team; It can remove unfavorable weather, set-up Tailwind for slower teammates to sweep, and clear hazards on the field. However, it doesn't have the best attacking moves, and lacks in power quite a lot. It is also not that bulky, and might be fainted easily.
Mega Rotom-C
Typing: Electric/Grass ----> Electric/Grass
Abilities: Levitate ----> Sap Sipper
BST: 50/65/107/105/107/86 ----> 50/115/117/115/112/111
New Moves: Wild Charge (all formes), Seed Bomb, Energy Ball
While regular Rotom-C's niche is to absorb paralysis and powder moves, Mega Rotom-C not only is still immune to such moves, it can even gain a boost in Atk from the powder moves! Being a mixed attacker, Mega Rotom-C has decent STAB moves on both sides, but the lack of boosting moves does impede it. Mega Rotom-C is the only one out of the Rotom formes which can possibly utilize its only priority move, Sucker Punch, decently, though.