It's good as a special wallbreaker down in NU. It is most definitely not the best Rotom form, however.
It's pretty good as a defensive Pokemon in the lower tiers, with a nice typing and access to Will-O-Wisp, Pain Split, and Defog. It functions well as a hazard clearer, being immune to three of the major four entry hazards. Again, however, not the best.
It can be a nice wallbreaker or support Pokemon, albeit in the lower tiers like the two forms before it. It has low Speed and a plethora of weaknesses, however. You're pretty much required to run Heavy-Duty Boots. It also needs to get a Nasty Plot boost to be good, tricky with its typing and Speed.
It performs an offensive support role (like just about every Rotom form) in the lower tiers (again), with access to things like Will-O-Wisp, Defog, Leaf Storm, and Volt Switch. Its typing makes it weak to common Pokemon like Ninetales, though, so there's usually a better option.
One of the two somewhat good Rotom forms. Its typing allows it to resist the multitude of ever-present Fairy and Steel types, and fire back with powerful Overheats and Thunderbolts. The downside of it being a Fire type, however, is that Heavy-Duty Boots is all but required for it to be good, especially if you plan to be Volt Switching in and out. It functions well as either an offensive Pokemon (Overheat, Thunderbolt) or a supporter/pivoter (Pain Split, Volt Switch, Defog).
Debatably the best Rotom form, contested with Rotom-H. Its Electric/Water typing paired with Levitate makes it weak to just Freeze-Dry and Grass moves, making it an excellent typing defensively. It unfortunately gets no recovery outside of Pain Split, making its bulk somewhat less usable, as it's prone to being chipped down. Nevertheless, Hydro Pump, Thunder, Volt Switch, Will-O-Wisp, and Thunder Wave are very good moves to have, and Rotom-W will more than likely put up a good fight on the field.
The best is probably either Wash form or Heat form, judging by what I've seen.
Hope I helped!