These are some old style and new age Harem series not to be confused with shows that happen to have a Harem. Harem is the driving force of the plot to some degree in all of these or some other plot device where the attraction of virtually all female characters toward the male lead is key to the story. This is not to be confused with harem usage in other genra such as Index or SOA where while present harem is not a key part of the plot but is used to support and build the value of the main character. I will however list some that fall under the dating sim category in witch the lead character has multiple characters to select between.

Maburaho
Nagasarete Airantou
Highschool DxD
Fortune Arterial: Akai Yakusoku
yosuga-no-sora (warning extreme content)
Highschool of the Dead
Amagami SS
MM!
Infinite Stratos
Baka to Test to Shoukanjuu
Lotte no Omocha!
Hoshizora e Kakaru Hashi
Hidan no Aria
Ben-To
Boku wa Tomodachi ga Sukunai
Maken-ki!
Koi to Senkyo to Chocolate
Campione! (despite new it has a more c;lassic feel to it (not to say it is like amazing like a classic))
Seitokai no Ichizon
Sakurasou no Pet na Kanojo
Maji de Watashi ni Koi Shinasai!!