Here is the full list of all 21 Plants vs. Zombies GOTY Edition achievements. It takes around 40-50 hours to unlock all of the achievements on Windows.
Defeat 2 Gargantuars with Corn Cob missiles in a single level.
Grow the Tree of Wisdom to 100 feet
Enable Mustache Mode
Get to 20 flags in Survival Endless
Discover the top secret zombie
Get the golden sunflower trophy
Get 8000 sun during a single level.
Collect all 49 plants
Get to a streak of 10 in I, Zombie Endless
Complete a nighttime level without planting any Mushrooms.
Get to a streak of 15 in Vasebreaker Endless
Beat all 20 mini games.
Bowl over 5 zombies with a single Wall-Nut.
Complete a daytime level by planting only Mushrooms and Coffee Beans.
Complete adventure mode
Defeat a normal roof level without using any catapult plants.
Complete a daytime pool level without using pea shooters of any kind.
Hypnotize the lead dancer zombie
Pick up 30 coins in a row on a single level without letting any disappear.
Blow up 10 zombies with a single cherry bomb
Blow up a zombie using a potato mine