G5 is a good one, my friend told me so. it's not based on a real gun, sadly. it resembles CZ's skorpion evo 3, but in 5.56 instead of 9mm. if you fancy something more realistic, KJW M4 is a good gun to start. not overly expensive, but it's actually good. really durable. my friend had owned it for 2 years until the he needed any replacement, and it's just the bolt. gas use is fair enough, stable ROF, and not overly high FPS.
but take my word: NO VFC
VFC's AEGS are known for superb quality and finish, and their GBB/R also has the the amazing finish and extremely realistic, with poor quality. parts are hard to find once you broke it. if you're new, avoid VFC. it's only for those with experience and money.
KSC/KWA kriss vector is also a good one. not a bad choice. or maybe you want to try Marui's new M4 MWS?