LH738

I assume you arrive at "Wien Westbahnhof" (there are also connections to Südbahnhof and to Hütteldorf).

You can use underground U3 from Westbahnhof to Stubentor ( http://www.wienerlinien.at/media/fil...utsch_3288.pdf ) and connect from there to tram no 2. Tram 2 stops in front of the Imperial ( http://www.wienerlinien.at/media/fil...plan_10966.pdf ). The walk from the train station to the underground is a bit longish though.

I'd also recommend the above mentioned Vienna card (if you intend to go e.g. into museums) or a 24/48/72h public transport ticket - depending how long you are going to stay. You can buy these tickets at Westbahnhof already.
You can do a lot by just walking but I found it comfortable to just jump into a tram to drive one or two stations. Vienna has a very good transport system with high frequencies (so this is quite a good alternative to the hop on/hop off sightseeing bus although you have to be your own tour guide without explanations/hints).
Trams 1+2 used to drive around the inner city with nice views but this is not the case anymore. Now you have to use both in order to do this tour and switch from line 1 to line 2 or vice versa.

Taxi is about 15-20EUR. I prefer the taxi if I have heavy luggage or >2-3 pieces as the walk to the underground is a bit longish (though there is also quite a walk to the taxi stand) and it's a bit of dragging oneself up the stairs in the older trams. But without much luggage, it's absolutely no problem to use public transport.

I don't know all the stations of the hop on hop off sightseeing bus and how many operators there are. There is at least one operator that stops in front of the opera which is a five minute walk from the Imperial ( http://www.viennasightseeingtours.com/index_eng.htm ).