SIL Gentium lacks some critical marks if viewing e.g. the SBL Greek NT, and has no bold face, but it's the most attractive (to my eye). On linus install with sudo apt-get install ttf-sil-gentium or similar.
For completeness, Cardo, Athena New Unicode or SBLGreek can't be beaten, though SBL is the least readable or attractive by some margin (again, to my eye).
For typesetting, with excellent Bolds and Italics, use the numerous highly professional Greek Font Society faces: e.g. GFS Bodoni or GFS Elpis for a serif or GFS NeoHellenic for (finally) a decent sans-serif.