I have a choice between the following tours: 1. Belvedere Palace & Leopold Museum
2. Liechtenstein Museum or 3. Imperial Vienna. Please Advise

It depends on your interests. If you want to see the Klimts, then definitely the Belvedere (and who knows how long they'll still be there). If you're a fan of Baroque, then the Liechtenstein Museum would be the perfect choice. As for Imperial Vienna, what exactly does that tour encompass? There's "Imperial" stuff all over town.

By the way, why would you need a tour of the Liechtenstein Museum, unless you're talking about a guided tour given by the museum itself?
I've been there several times, it's very self-explanatory. With your admission fee, you receive a booklet (available in several languages) which has a nice, fairly detailed descriptions of each painting (by room) and there are also brochures explaining the history of the museum. They offer very nice concerts on Sunday afternoon in the largest room upstairs (free with museum admission).

Getting to the museum is easy as well, it's about a block from a stop on the D tram. (The D tram also stops right in front of the upper Belvedere.)

Thanks for your suggestions, especically about The Liechtenstein. The "Imperial" tour included the Karnterstrasse, the Antique Quarter and Hofburg Palace. What do you know about the Emperor's Pavillion and the Lueger Church? Comments on any the of the above sites would be appreciated.

Truthfully, not much. Of the three tours, probably the last would be the best value. You could manage easily in both the Belvedere and Liechtenstein museums without a guided tour. Just bring a good guidebook along to supplement the info available at the museum (most applicable to the Belvedere--the Liechenstein Museum, while in a beautifully restored historic palace, is too new as a museum for the guidebooks to have much info that isn't available at the museum itself).