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Whilst I’m not a great fan of either Helvetica or Arial, this article on the subject of Arial’s displacement of Helvetica means that I view Helvetica slightly better than before. Certainly in a better light than Arial.
My favourite fonts at the moment would be the Bitstream Vera series ([get them here](http://www.gnome.org/fonts/) ) and Trebuchet MS. On the serif side, I still like Georgia, especially when xft'd. Non-anti aliased it looks a little thin. Garamond, the old classic Apple font, is also one I like on certain occasions. For monospace fonts, Bitstream Vera Sans Mono is my favourite **by far** right now. It's got a quality and care to its look I haven't seen for a long time in monospaced fonts.
Recently XFT, the Linux font renderer, has improved in leaps and bounds. I would say that it now beats Windows font rendering hands down. For those that deride Linux font handling, grab yourself a new distro with XFT2, copy your Windows fonts into ~/.fonts (along with the Bitstreams, of course), and prepare for a nice surprise =)