Well that question is definitely an AFFC (alltime favorite forums classic ) and I love it! Although my answer has not changed since it was asked here for the first time. I miss Horizons the most, I am very sad I didn't have the chance to make a GOOD QUALITY video of the ride and still consider it a grave mistake, they didn't give this epitomic ride in Future World a second chance, instead demolished it to build a half-a..ed approach to what would have been a truly epic successor but was downsized to a "training mission" instead, Mission Space. BTW, everyone who wants to learn about what Horizons was like should watch the truly amazing video by Martin on Youtube!
I understand that Horizons was a little dated in the 90ies but it was still very popular as well.
I also miss the Barker Bird at the Tiki Room, the firing cannons and steeldrum bands at Carribean Plaza, all these added a lot to the atmosphere in Adventureland.