Everything by Jun Maeda. That would be Angel beats, Clannad (After Story), Kanon, Air.
But it also really depends on if you want romance harem shows or if you want just 1 guy and girl. Keep in mind that for romance harems the guy will in the end pick 1 girl so it is still romantic and gooey but there are other girls in the side as well. I have ordered each of these lists by how much romance and gooeyness I feel are shown.
Other great harems would be
Mashiro-Iro Symphony: The Color Of Lovers
H2O: Footprints in the Sand
Hoshizora E Kakaru Hashi
Tayutama: Kiss On My Deity
School Days (be careful here)
And other romances that are not harems
Ef - A Tale Of Memories (Melodies)
Ano Natsu de Matteru
Tonari no Kaibutsu-kun
Chuunibyou Demo Koi ga Shitai!
Zanz Bonus Points Tally
8bit_samurai: 1, CodeZTM: 1, Eake4: 2, ibabrak: 5, Mirkosp: 4, tipsd9video: 1, uncreative: 1