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