Welcome to the forum dude!
Based on my own reading and browsing, the 3.6L appears the most reliable, followed closely by the 3.5L. The 2.4L I4 and the 3.9L seem to be where the troubles arise. Being a power hog with an efficiency streak, I always steer clear of naturally aspirated (non-turbo) 4 cylinder engines, and have a mistrust for high-displacement V6s like the 3.9L.
Personally, I think the 3.6L engine gives you the most bang for your buck, being a dual-overhead cam design rather than a pushrod design. It's a more modern, energy efficient valvetrain configuration that allows for higher performance and better fuel economy (depending on other factors, of course). I just got myself a 2008 G6 GXP with that particular engine, and I really like it. Lot of power and pickup, great passing power in all speed regimes, and runs very smooth.
Last edited by C0RSA1R; 02-18-2014 at 09:13 AM.
Reason: forgot to welcome the dude