The folks at Volta (Avanti's apparel side) have been making a few decent garments for the MTB set over the last year or two. But with this new Tech Tee they seem to have stepped it up. The top has obviously had input from an actual mountain biker, and luckily it's one who likes wearing semi-loose garments but wants to ride all day (and not an XC rider that thinks they know what's up). I've been wearing this top for the last few months and have been really happy with it. Like all synthetic tops it needs a wash after every ride unless you live and ride alone, but the length, breathability and graphics all get top marks.
You can pick one up at your local Avanti Plus store, but for some reason it's not on the web store yet.