I've only played 2 so far ( planscape torment and Icewind dale) so I don't have much in the way of an opinion other then planscape is better then anything in forgotten rhemls could be. Note since I liked Arcanum I may get TOEE despite the bugs and campy graphics iv'e herd so much about.

Worst to best of those I've actually played:

Buck Rogers: Matrix Cubed
Gateway to the Savage Frontier
Secret of the Silver Blades
Icewind Dale 2
Buck Rogers: Countdown to Doomsday
Baldur's Gate (vanilla)
Champions of Krynn, Treasures of the Savage Frontier
Pool of Raidance, Curse of the Azure Bonds, Death Knights of Krynn
Pools of Darkness
Dark Queen of Krynn
Icewind Dale
Baldur's Gate (modded)
Baldur's Gate 2

In that case play Planscape torment. You won't regret it (although I sugest modding it from the get go)