There are two sets that I think should be the standards for M!Ampharos
BECAUSE YOU'RE WORTH IT
Ampharos@Ampharosite
Modest/Static
252Hp/252SpAtk/4SpDef
-Volt Switch
-Dragon Pulse
-Focus Blast
-Hidden Power Fire/Hidden Power Ice
Overview
This Ampharos is played as a defensive pivot. With it's great bulk and awesome typing it can come in on quite a number of pokemon. Most importantly is Rotom-W who's STABS fail to do any real damage and who's Volt Switch you quad resist. M!Ampharos also resists both of Talonflame's STABs giving you ample ability to switch in and scare it out. Focus Blast is for coverage nailing things like Heatran and TTar on the switch. The 4th move is pretty much open, Hidden Power Fire is for Forretress, Ferrothorn and Scizor and Hidden Power Ice is for Landorus(T) and Gliscor, though it's worth noting only Ferrothorn can reasonably switch in on either of Ampharos' STAB moves.
Pivot Ampharos absolutely loves Rotom-W and Gyarados as partners. Both are immune to ground and both beat common Ampharos switch ins like Heatran, Tyranitar, and Gliscor. Gyarados handles Ferrothorn and Blissey better than Rotom does with taunt however Rotom lacks a stealth rocks weakness allowing it to switch in more freely.
MAYBE SHE'S BORN WITH IT
Ampharos@Ampharosite
Modest(Quite)/Static
252Hp/252SpAtk/4SpDef
-Substitute
-Thunderbolt
-Dragon Pulse
-Focus Blast/Focus Punch
Unlike the previous Amparos which is made to take a hit and scout, Sub Ampharos is made to deal some real lasting damage. Ampharos is great at forcing switches from two of the absolute most common pokemon in the game in Rotom-W and Talonflame freeing it up to substitute. With a sub up Ampharos can more easily deal with common switch ins. While Double STAB and Focus blast gives great coverage, Focus Punch is also a great option for beating out the likes of Blissey and Chansey, though if you go with the Sub Punching set you should use an EV spread of 252 HP/160 Atk/96 SpAtk with a Quite nature. This allows for 50% minimum on Chansey with focus punch and still leaves you with a massive 429 SpAtk. Be warned however that Focus Punch Ampharos actually loses to Rocky Helmet Ferrothorn as even though Ferrothorn can't deal any real damage to Ampharos, you can't KO it without taking massive damage yourself.
Outside of these two sets Ampharos cant really do anything well. Even with it's great bulk and good typing it cant make a reasonable tank due to it's lack of recovery. There's other options for the 4th slot in the pivot set as well: Heal Bell can be used to help support the team and rid it of status, Signal Beam can be used for a OHKO on Celebi. Power Gem should be ignored as it's coverage is redundant only hitting Rotom-H harder than your other three coverage moves. Ampharos also gets light screen, which can support the team and help it beat out random Ice Beam users however without access to light clay it's not a solid option.
Counters:
Anything with STAB earthquake that can survive a hit from ampharos is a counter. Excadrill gets special mention for being immune to electric attacks and resisting Dragon attacks but has to be weary of Focus blast or Substitute on the switch. Florgres with it's massive Special Defense and dragon immunity can handle all of Ampharos' attacks well and hit back with STAB fairy attacks or Toxic. Chansey and Blissey are a 100% counter to Ampharos that lacks Focus Punch. Gliscor and Landorus(T) fear only Hidden Power Ice but can survive a Dragon Pulse and hit back with Earthquake. Dugtrio has trouble getting in but can trap and OHKO with Banded Earthquake.
Sorry for the less than full write up, Im on a bit of a time limit, I can edit in more depth later.