It all depends what you want from your gun.
G&P- Amazing externals, decent internals.
G&G-Externals are okay, Internals are great for their price.
ICS- Their build quality is good, but they are notorious for being hard to fit with non ICS externals due to their non standard measurements. Their internals are very easy to work on however due to their split gear box system.
Those are the only brands I've had personal hands on experience with, if I had to chose it would probably come down to G&P for realistic externals, I'd then spend money upgrading the internals when and as needed.
For example you could get a G&P M4 or M16 for £230, then use the remaining 130 to get yourself some nice upgrades.