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