"Best" is subjective.

You might look at WCF, which gives you better future flexibility.

Check
http://sholliday.spaces.live.com/Blog/cns!A68482B9628A842A!158.entry

or maybe look at Astoria
http://www.microsoft.com/downloads/details.aspx?FamilyId=1B6F85BC-8933-4D0E-A639
-934EF85ADCE1&displaylang=en

I have a web application that calls/uses/consumes WCF services.

This is what i think..

It will depend on many factors.  like where will your c#  app going to
run? is this an intranet application or should it also work from your
home (without vpn)

I am assuming that you are going to use Stored procedures with  Data
access layer. with the web service/WCF.

If it is intranet app,
you will get better performance if you use your app + Stored
procedures with  Data access layer
If your web application will be Asp.net application then you can re-
use the "Stored procedures with  Data access layer"
but if you have some other technology you will better off with the web
services + Stored procedures with  Data access layer approach.

if your app is going run over internet, then web services/wcf approach
would be better

You should only expose your DAL as web service in very special
circumstances.

You should expose your BLL as web service if you foresee a need to
use it from apps written in different technologies/languages.

No. See above.

You could.

But the decision whether to use SP's or plain SQL in your DAL is not
related to the interfaces provided by DAL and BLL.

See above.

Web services and stored procedures can not replace each other so
advantages/disadvantages between them does not have any meaning.

Arne