Magneton is ridiculously good - steel/electric typing, three viable abilities, multiple sets (although they all use the same moves, so it's more like multiple items), lots of power, solid physical bulk with Eviolite, and plenty of power behind its special attacks.
With Magneton's introduction into the tier, a lot of people immediately resorted to using Stunfisk or Lanturn. Although part of this shift was due to Seismitoad's departure from the tier, the introduction of Magneton and, to a lesser extent, Fletch were primarily responsible for this trend.
Sure, both of these pokemon can laugh at any STAB attack Magneton throws at them, but it does run HP Ground very often and with analytic (I know prediction is a two-way street and predicting the switch-in isn't too consistent an argument, but it's usually obvious as they're reliable switch-ins generally), HP Ground can 2HKO offensive Lanturn (specs or agility) and at least put a dent into bulkier variants before likely being forced out. Now I know that these two are still easily considered checks, but wearing Lanturn or Stunfisk out isn't too challenging in this metagame, so it is fair to say that there really isn't any 'super reliable consistent' counter to Magneton outside of the uncommon (and imo lackluster) Restalk Lanturn. Also, I prefer analytic over either other ability as it makes Magneton insanely powerful when hitting a switch-in or moving second, even if it is Eviolite instead of Specs, but Magnet Pull is viable, although situational, for trapping Ferroseed (need to be HP Fire or SubChargeBeam) or Klinklang, and Sturdy is viable, too (just ask Omfuga).
Going back onto the main line of thought, outside of the two aforementioned answers to Magneton, in Lanturn and Stunfisk, there aren't many other checks/counters. Electevire and Raichu both absorb any electric move and resist Flash Cannon, sure, but they don't like Specs or Analytic Flash Cannon while HP Ground will do a ton. I would classify them both as offensive checks, but Elect is the only one of the two that is decent currently and even then, I don't see to many people using it. AV users like Hariyama and Magmortar can defensively check it through taking 2-3 hits before being low or dying, but it can easily volt on the switch in or, if facing Hariyama, volt into a check for some more damage (not to mention neither appreciates specs or even Eviolite attacks when boosted by analytic when they are switching in as neither resists electric, so they get worn out easily and cost their own team momentum). Honorable mention goes to SDef Zweilous for being a specially defensive behemoth, but it does nothing in return while still taking enough to be threatened by Flash Cannon (so it's not a check at all), and Ferroseed, which is easy volt switch bait for any set, but can defensively wall any non-HP Fire variant (although Magnet Pull HP Fire gives it the D, so be careful) - too bad it gives up all momentum and does very little in return.
Overall, Magneton has a lack of checks/counters that makes me feel the need to run one of a select few things in teambuilding which restricts me quite a bit while it can use Specs, Eviolite, or Scarf with Analytic, Magnet Pull, or Sturdy to fulfill various roles and function as you'd like it to. I think a ban is appropriate.
