Spring:

Moondrop:
Seeds bought from Fantastic Flowers for 70 gold.
Sells for 600 gold at level 1.

Summer:

Fireflower:
Seeds bought from Fantastic Flowers for 260 gold (Use gold saved up from selling moondrops)
Sells for 1,200 gold at level 1.

Autumn:

Carrots:
Seeds bought from the General Store for 100 gold.
Sells for 590 gold at level 1.

Winter:


Leeks:
Seeds bought from the General Store for 120 gold.
Sells for 500 gold at level 1.

Spring:

Emery Flower:
Seed bought from Fantastic Flowers for 1000 gold (After completing the 4th dungeon)
Sells for 12,800 at level 1.

Summer:
Green Crystal:
Seeds bought from Fantastic Flowers for 700 gold (After completing the 4th dungeon)
Sells for 7,000 at level 1.

Autumn:
Red Crystal:
Seeds bought from Fantastic Flowers for 700 gold (After completing the 4th dungeon)
Sells for 7,000 at level 1.

Winter:
White Crystal:
Seeds bought from Fantastic Flowers for 700 gold (After completing the 4th dungeon)
Sells for 7,000 at level 1.

I'll update the guide as I play if prices have been changed.

At high pharma skills, you can use the crystal flowers to permanently boost your stats.

At high pharma skills it's better to grow clovers in the summer (Sold for 390 gold at Fantastic flowers after beating the 4th dungeon) As clovers can be used to craft Levelizer.

If you have a pharma skill of 82, you can grow Autumn Grass and make Invinciroids, which sell for 45,620 each!