Blissey@Leftovers/Heat Rock
Modest
Serene Grace
252 Sp.Atk/252 HP/4 Def
~SolarBeam
~Flamethrower/Fire Blast
~Focus Blast/Sunny Day
~Shadow Ball/Sunny Day
Blissey can function as a sun sweeper, if only through the element of surprise. Blissey has a lovely special movepool, which includes the 1-turn-in-sunlight Solarbeam, and a handful of Fire moves, which gain power in sun. This allows Blissey to function rather well in the sun. Focus Blast and Shadow Ball provide additional coverage, although Sunny Day can be subbed in there in the event of your foe bringing in an Abomasnow or Tyranitar or something after taking out your Ninetales.
Serene Grace is the ability of choice here, if only to boost the chance for Blissey's chosen Fire-type move to burn the foe, lessening any physical damage Blissey takes. If you're running Sunny Day, go with Heat Rock. Otherwise, go with Leftovers.
Blissey@Leftovers
Adamant (don't laugh)
Natural Cure
252 Atk/252 HP/4 Def
~Cheer Up
~Egg Bomb/Return/Frustration
~Earthquake
~Focus Punch/Brick Break/Drain Punch
This generation gave Blissey a decidedly gimmicky new toy in the guise of Cheer Up. Blissey's only means of boosting its pathetic Attack stat. Now, due to its relative bulk, Blissey just might be able to pull off a Cheer Up or two. During this time, an opponent will inevitably bring in their special wall, clearly thinking you're intending to use special attacks, as nobody would be crazy enough to use a physical attacking Blissey. And that's when you slam them down with a physical attack. Egg Bomb is a decent option, almost exclusive to Blissey, but it lacks accuracy, and is less powerful than Return or Frustration would be. Either move would work, but for those of you who do this ingame, Return would obviously be easier, given the need to max out Chansey's happiness to evolve it. I would've included the more powerful Double-Edge here, but all that recoil damage on a defensive Pokemon? Not exactly the best idea. Earthquake provides its usual power and coverage, but beware of balloon holders. The final move is up to you. Focus Punch can catch opponents switching in, but if you're not that good at prediction, go with Brick Break. Drain Punch is also an option if you're not liking Leftovers recovery alone.
Natural Cure is the ability you'll want, as it's really the only one that'll actually have a discernable use here. None of these moves get boosted by Serene Grace, and as this is intended for single battles, Healing Heart would be useless. As for the item, go with Leftovers, give Blissey a little bit of recovery.
I think it goes without saying, but you really have to possess huge quantities of guts and/or insanity to even consider using this Blissey. But I guarantee you, no opponent will ever see it coming. So if you're one to take advantage of the element of surprise, go right ahead.
Blissey@Leftovers/Light Clay
Bold
Healing Heart
252 Def/252 Sp.Atk/4 HP
~Healing Beam
~Helping Hand/Swagger
~Reflect/Healing Wish
~Light Screen/Healing Wish
Use this one in Doubles and Triples. Healing Beam heals your teammates. Helping Hand powers up your teammates. Reflect and Light Screen provides a bit of extra bulk for your team. Alternatively, Swagger can be subbed in place of Helping Hand if you have a physical sweeper with Own Tempo, give it a more lasting boost than Helping Hand. You can also replace a screen with Healing Wish, should you deem it necessary.
Healing Heart is the ability of choice here, as this Blissey is more about helping out its teammates. As for the item, up to you. Leftovers are pretty much self-explanatory. However, if you want the screens to hold up a bit longer, go ahead and use Light Clay.
Blissey@Leftovers
Modest
Serene Grace
252 Sp.Atk/252 Def/4 HP
~Charge Beam
~Ice Beam
~Shadow Ball
~Focus Blast
All-out special attacker. With Serene Grace, Charge Beam is an automatic Special Attack increase with every use. Pair that with Ice Beam for the somewhat dreaded BoltBeam combo. Or BeamBeam in this case. Shadow Ball and Focus Blast provide perfect coverage.
Given the usage of Charge Beam here, Serene Grace is the obvious ability. As for the item, go with Leftovers. Why go with an item that ups your Special Attack and restricts you to one move when Charge Beam accomplishes that without the restriction, while leaving you free to heal a bit of HP each turn?
Blissey@Leftovers
Adamant (don't laugh)
Serene Grace
252 Atk/252 HP/4 Def
~Cheer Up
~Thunder Wave
~Headbutt
~Rock Slide
Dunsparce this ain't, but that doesn't mean it can't try. Anyone who's faced (and especially anyone who's lost to) a Dunsparce in competitive battle (and I know for a fact that there are such people out there, as I myself have earned a decent number of victories with a Dunsparce) knows of the strategy known and feared as FlinchHax. Dunsparce does it best, with the likes of Togekiss and Jirachi also doing it well. But why not Blissey? Well, I'll tell you why, that pitiful base 10 Attack. But that's where Cheer Up comes in. Boost Blissey's Attack to at least a respectable amount, T-wave the foe, then flinch the hell out of them with Headbutt. Once the foe faints, just T-wave the next victim and repeat the process. Assuming your foe doesn't ragequit before the afflicted Pokemon faints, that is.