Little Busters! EX was awesome, as most of Key's eroges they have a minimal but good amount of sex scenes so it doesn't take your mind off of the plot, the game play is very fun, the characters are splendid, and the soundtrack pwns.
Another good one, and an obvious classic would be School Days, the most screwed up visual novel (besides bible black and saya no una) ever made. and the endings all so fun
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Clannad, it has great characters and an amazing story, plus an afterstory as your reward for completing all of the school life endings.
Muv Luv, its a bit cliche, the story is a bit half baked, and you can only make it with two girls, but its still funny the animation (or close enough to animation, after all it is a VN) is great, and unlimited is a awesome
Finally we have Kanon: Key's first visual novel, and one of the best visual novels ever. Its a bit simplistic and parts get boring, but the characters are all unique (Though cliche) and the story is fun. Mai for the win
