What is the best ready made buttercream that can be suggested either with or without fondant?
In my opinion, the manufacturer that sells the most is probably the best. That would be H.C. Brill... specifically, their Xtra Smooth (even though their Light 'n Fluffy is probably easier to get through most distributors). I used to work for a competitor of Brill, and believe Brill's product was probably superior at least in texture and appearance yet tastes the same as any commercial buttercream.
You can read these threads about Brill:
I edited the link three times... don't know why it doesn't work. The page is suppose to show five threads. If you go to Forums and press the Search button, then enter Brill in the subject box and CWR41 as the author, you'll be able to read the threads.
If you have a Sams Club close by than I suggest their icing. Also carry whipped bettercream and chocolate bettercream and buttercream as well.