1680 DUST D5 TO LEGEND BEST AND CHEAPEST AGGRO, IN DEPTH MULLIGAN

I believe this to be the strongest variation of treant Druid I also think this is probably the strongest aggro deck in the meta right now coincidentally this is also the cheapest one! Climbed to Dia 5 with my other for fun Deathknight decks, this took me from Dia to Legend with 3 loses one of which was due to a misplay.

Solar Eclipse + Plot of Sin = Turn 4 coin into win game cheese or an extremely strong play regardless

Solar Eclipse + Cultivation = 2 mana +4/4 to your minions is really balanced 

Solar Eclipse + Power of the Wild = Pseudo Cultivation 

Solar Eclipse + Forest Seedlings = Very strong turn 3 play if your seedlings already marinated

Summer Flowerchild = Draw into your cultivation, your big draw or unending swarm to restore board

Extensive mulligan guide:

Mulligan without coin: Always keep Living Roots, Lingering Zombie, Forest Seedlings, do not keep more than two 1 drops.  You can keep Natural Causes if you already have Aerosoilizer and a 1 drop. Keep Plot of Sin  if you have good early spam in your hand already for example any 2 of these Forest Seedlings, Lingering Zombie, Living Roots.  You can keep Life From Death if you already have a lot of tokens in your opening hand. Witchwood Apple is a good keep if you already have a one drop but you need to mulligan 2 cards away, pretty much keep this always if you are uncertain about having some of the stronger openings it fills out your curve really well. If you have Forest Seedlings and any turn 2 play, you can keep Solar Eclipse to play 4x 2/2 on your turn 3, this I would always keep if the 3rd card in your hand happened to be Cactus Construct.

Mulligan with coin: Always keep Lingering Zombie, Living Roots, Forest Seedlings but do not keep more than 2 of any of these cards.
Always keep Aerosoilizer as a coin play on turn 2. You can keep Plot of Sin if you have good early swarm also you can keep Solar Eclipse with your Plot of Sin as a coin turn 4 play, if you manage to infuse it with Lingering Zombie and other minions this is essentially game over on turn 4. You can keep Life From Death if you already have a lot of tokens in your opening hand. Witchwood Apple is a good keep if you already have a one drop but you need to mulligan 2 cards away, pretty much keep this always if you are uncertain about having some of the stronger openings it fills out your curve really well. Soul of the Forest  Is good keep if you think you can land it on 3 minions at least on turn 3 with coin, I would only keep this in a game where I am going second. Forest Seedlings + Solar Eclipse is also viable, but I do not pref. this that much as my coin hand

Note for both hands: It is better to keep Cactus Construct if you do not have Forest Seedlings ,Lingering Zombie or Living Roots in your hand as risking not having turn 1 play can throw the whole match, especially if you are going first, as if you are going second you can coin Witchwood Apple or hope to mulligan into other 1 drop a lot more consistently. 

Only state where you would ever keep Summer Flowerchild is if you were going second had Lingering Zombie  OR Living Roots AND Forest Seedlings AND Solar Eclipse , as you could cheese early Cultivation  or Unending Swarm  or potentially both by coining Summer Flowerchild at turn 4 and drawing into them. This is very viable but if you were to not draw a 1 cost card in between your curve can be incomplete and if you for example get hit with a Hellfire or an early boardclear and one of the two draws from Summer Flowerchild was not Unending Swarm it can be game over. 

Never keep regardless of coin: Cultivation , Unending Swarm, Power of the Wild and Summer Flowerchild except the example I provided.