Rotom-W is the best out of all imo
Why you ask? Rotom-W has the best defensive typing out of all of them, with a main weakness in Grass, thanks to levitate which lets it dodge Ground type attacks. IT has a strong dual STAB Electric + Water combination, and gets access to one of the most coveted moves - Will-O-Wisp, which makes it one of the few Pokemon able to counter threats like Aegislash, Talonflame and most physical attackers in general. This means that it is straight away part of many strong teams, due to it's versatility. Combined with it's strong bulk, and special attack it makes an excellent tank Pokemon.
The other rotom forms have their uses too, though Rotom-F and the Normal Form Rotom are probably the most useless.
If you look at Ampharos, it's also an excellent tank with an electric typing, as well as a Mega Stone giving it a Dragon/Electric typing as well as buffing up it's stats making it even better as a tank. It gets access to Heal Bell giving it the ability to support the team, but personally I would still nevertheless vote Rotom-W.
However if you're looking at Rotom-C (Rotom-Mow) it's a solid option, but not really as good as Rotom-W in most cases. While they have a fairly similar movepool, Rotom-C's dual STABs are resisted by Dragon and Grass types . However, it's still a solid option, and a core of Rotom-C + Heatran is an excellent one, so run it if you want.