Best Microsoft Office alternatives

Microsoft Office no longer has an unquestioned stranglehold on the world of productivity suites and programs. Though Office has expanded to other platforms, like Chrome OS, there are plenty of capable alternatives out there for Windows and Mac OS X.
With that in mind, here are our picks for the best Microsoft Office alternatives.

[TracTech]

Can someone test compatibility of this programs with .docx, xlsx, ... files? As I see non of this alternatives opens .docx as it should. Header and footer is missing, page numbers are missing, page brakes are gone, equations are as pictures (can't be modified), and so on. Why do they all talk about compatibility but the only thing that they can open is pure text.

[Deontjie]

Can someone intelligent combines the power and ease-of-use of Quattro Pro with the stability and price of LibreOffice?

[tyrmyrmidon]

BUT WHICH ONE SHOULD I USE DOT DOT DOT

[tunatezer]

I need an office suite for LINUX and with RIBBON.