You should provide what kind of location or travel budget that you are asking about: there's great conferences across Europe but that may not be of interest if you are based on the West coast of the states for example.
But love it here anyhow!
I went to linux.conf.au for five years in a row (from India).
Heard people like like Andrew Tanenbaum and Bruce Schneier, more importantly go to ask them questions (yes, that's the point).
Rusty, Tridge, Linus, Bdale, Keith Packard, Lennart, Garret, Val Henson, Rasmus ... if you work on the lower layers of the linux ecosystem, this is a pretty awesome place to head to.
The only downside was that I returned from the conference every year with a sensation of having wasted the entire year before not doing anything significant (of course, I worked on a lot of small things, but nothing which took a year to write).