I decided I'd have a go at an OU team, but the last time I played Blissey was the most used Pokemon, so the metagame must have changed a lot since I last played.
Metagross @ Choice Scarf
Jolly
28 HP/252 Atk/228 Spe
Meteor Mash
Earthquake
Ice Punch
Thunderpunch
I picked Metagross because I needed a dragon resist. It gets a few suprise kills thanks to its speed and brings lots of useful resists to my team. It also helps to finish Pokemon off that Mamoswine is too slow to kill. Grass knot and Hp Fire would be useful, but I don't think I have enough moveslots for it - I could put it over Thunderpunch, but that would give me Gyarados trouble, and I don't think Zapdos can take care of it. Maybe Trick could work too, but I like keeping a fast Pokemon for lategame.
Zapdos @ Leftovers
Bold
252 HP/220 Def 36 Spe
Thunderbolt
Heatwave
Roost
Hidden Power [Ice]
Zapdos is a good physical wall on my team, and brings a Ground immunity to my team. I tried with Light Screen over hidden power, but then I couldn't deal with Ground Pokemon, but without Heatwave Steels were an issue, so I put both. I was thinking Discharge over Thunderbolt, I don't know if that would to too weak to kill certain Pokemon or something, but it would give me some Paralysis support that Porygon-Z needs.
Hariyama @ Leftovers
Impish
76 HP/180 Atk/252 Def
Thick Fat
Knock Off
Force Palm
Ice Punch
Whirlwind
My secondary Physical wall and knock off support. He also helps by bringing fire and ice thanks to thick fat. Force Palm has a higher paralysis rate, and doesn't have defensive drops like Close Combat. Whirlwind helps to get rid of stat raisers and baton pass chains, but unfortunately does not build up entry hazard damage because I don't have anyone with Stealth Rock on my team.
Blissey @ Leftovers
Calm
252 Def/80 SpA/176 SpD
Wish
Protect
Flamethrower
Seismic Toss
Special Wall. Standard WishBliss, who I thought would be useful because I have two Choice users, but I don't particularly like it though, especially because I can't touch Heatran. Would the "Diverse Blissey" set work better? I'm unsure about thunderwave and toxic, because paralysis support would further help Porygon-Z, but Toxic helps to get bulky waters which can be annoying.
Mamoswine @ Choice Band
Adamant
64 HP/252 Atk/192 Spe
Earthquake
Stone Edge
Ice Shard
Superpower
Physical Sweeper, who packs more power opposed to Metagross who is more of a lead/finisher. Its good to have a prioity move on my team, especially one that can kill Skymin. I thought I'd try out Superpower, the new Platinum move, but I haven't really found any situation where it is useful. I would like to swap this to lessen my team's fighting weakness, and because I don't really like it, but don't know what else would be good here.
Starmie @ Leftovers
Timid
160 HP / 132 SpA / 216 Spee
Thunderbolt
Rapid Spin
Recover
Surf
Put over Porygon-Z

Metagross @ Choice Scarf
Jolly
28 HP/252 Atk/228 Spe
Meteor Mash
Earthquake
Ice Punch
Thunderpunch
I picked Metagross because I needed a dragon resist. It gets a few suprise kills thanks to its speed and brings lots of useful resists to my team. It also helps to finish Pokemon off that Mamoswine is too slow to kill. Grass knot and Hp Fire would be useful, but I don't think I have enough moveslots for it - I could put it over Thunderpunch, but that would give me Gyarados trouble, and I don't think Zapdos can take care of it. Maybe Trick could work too, but I like keeping a fast Pokemon for lategame.

Zapdos @ Leftovers
Bold
252 HP/220 Def 36 Spe
Thunderbolt
Heatwave
Roost
Hidden Power [Ice]
Zapdos is a good physical wall on my team, and brings a Ground immunity to my team. I tried with Light Screen over hidden power, but then I couldn't deal with Ground Pokemon, but without Heatwave Steels were an issue, so I put both. I was thinking Discharge over Thunderbolt, I don't know if that would to too weak to kill certain Pokemon or something, but it would give me some Paralysis support that Porygon-Z needs.

Hariyama @ Leftovers
Impish
76 HP/180 Atk/252 Def
Thick Fat
Knock Off
Force Palm
Ice Punch
Whirlwind
My secondary Physical wall and knock off support. He also helps by bringing fire and ice thanks to thick fat. Force Palm has a higher paralysis rate, and doesn't have defensive drops like Close Combat. Whirlwind helps to get rid of stat raisers and baton pass chains, but unfortunately does not build up entry hazard damage because I don't have anyone with Stealth Rock on my team.

Blissey @ Leftovers
Calm
252 Def/80 SpA/176 SpD
Wish
Protect
Flamethrower
Seismic Toss
Special Wall. Standard WishBliss, who I thought would be useful because I have two Choice users, but I don't particularly like it though, especially because I can't touch Heatran. Would the "Diverse Blissey" set work better? I'm unsure about thunderwave and toxic, because paralysis support would further help Porygon-Z, but Toxic helps to get bulky waters which can be annoying.

Mamoswine @ Choice Band
Adamant
64 HP/252 Atk/192 Spe
Earthquake
Stone Edge
Ice Shard
Superpower
Physical Sweeper, who packs more power opposed to Metagross who is more of a lead/finisher. Its good to have a prioity move on my team, especially one that can kill Skymin. I thought I'd try out Superpower, the new Platinum move, but I haven't really found any situation where it is useful. I would like to swap this to lessen my team's fighting weakness, and because I don't really like it, but don't know what else would be good here.

Starmie @ Leftovers
Timid
160 HP / 132 SpA / 216 Spee
Thunderbolt
Rapid Spin
Recover
Surf
Put over Porygon-Z