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http://www.airsoft-forums.co.uk/index.php/topic/28392-aeg-or-gas/
http://www.airsoft-forums.co.uk/index.php/topic/28350-new-to-the-sport-gas-gun-advice-wanted/
To answer your question: GHK if you can afford it; the G5 appears to be basically the most reliable GBBR (a lot to do with their well revised magazine design) out there other than possibly something like the TM MP7 (though that's not a full length rifle, obviously). Tippmann (i.e. the M4) is pretty well reviewed too.
In addition though it's worth saying that GBBRs are, as a rule, not as forgiving as AEGs for being unloved. Although arguably they're harder to break a lot of the time (what with being fully mechanical), there are lots of bad brands, magazines are expensive plus maintenance does need to be done fairly regularly to stop leaks. If you're looking for something easy, a GBBR might not be it but certainly the GHK and Tippmann stuff is pretty damn reliable right now compared to what was about 5 years ago.
Also we're going into winter and GBBRs are pretty naff from November until February in my experience. Expensive, high-pressure gasses or HPA tapped magazines are a way around this, but that's another story.