I've played a decent amount of VNs before.
The longest VN for me is CLANNAD. The story is great - I get more feels from the VN than the anime.
The others that I remember are...
Katawa Shoujo (Good story, some ero)
Kindred Spirits on the Roof (yuri. I don't even know why I play this. Pretty good if you like reading.)
Roommates (Swings three ways, some ero, pretty good)
Beach Bounce (Mostly just eroge)
Cursed Sight (Great story, no ero)
X-Note and Area-X (sequel) (Mystery, cute graphics, no ero, great game)