What's the easiest plant to grow in Summer ?

Nasturtiums. Little bit of damp soil, little bit of heat, and within a week or two you'll have a vine covered in flowers. 

Mostplants ARE easy to grow if they have the correct growing conditions...soil, sun/shade and water is all they need.

Veggies are easy to grow: like tomatoes,  cucumbers, bean vines fresh herbs.

Flowers like marigolds, coleus, impatients and begonias are easy to grow. So are butterfly bushes and lilac trees and Rose of Sharons. 

easiest , the common weed , they grow anywhere .

It depends on where you are. cactus does very well in Arizona while oak trees do quite well in Illinois.

Poison ivy seems to be the easiest plant to grow!

Plant some Hostas.

Crabgrass. Just try it once--and you'll see.

Day Lillies                         

Weed. Not the one you smoke but the one that grows everywhere. It’s the bane of the brother in law’s existence. He’s trying to make a nice yard to outdo his neighbor so whenever a weed pops up, it annoys the hell out of him so he has to go buy weed killer then more grass seeds to grow back in that bald patch. 

I grow a whole lot of peppers (jalapeños, habaneros, Thai chili, Serrano) on my balcony. 

Spearmint and peppermint are also easy to grow. If you ask me, they’re just flavorful weeds. 

Basil is also easy to grow. 

Probably sunflowers.