It depends on what you’re looking for really.
In the world of Airsoft, there is no one perfect company or manufacturer.

Each brand has their strengths and weaknesses.

Firstly I would set a budget.
Be a little flexible but this is there to stop you spending more than you can afford.

Your geographical location matters as well.
Tokyo Marui guns are quite cheap in Japan but two to three times more expensive in the UK.

I’ll try to list a few common brands and what to look out for in each one:

Tokyo Marui
Pros
-excellent quality control
-superbly designed
-great bodywork on their new lines
-legendary hop chambers
-excellent range of upgrades
-lots of features on some models (recoil, empty mag detection)

Cons
-low power
-can be expensive
-mediocre stock performance

G&P
Pros
-fantastic body work
-solid gearbox shell
-standard v2 = lots of compatible upgrades
-make great bases to work on
-huge range of guns to choose from

Cons
-terrible stock motors
-mediocre quality internals

VFC
Pros
-excellent bodywork
-lots of features (working bolt lock etc)
-reasonable quality internals
-some models make great bases to work on

Cons
-generally more expensive than G&P
-some models have proprietary internals
-very mixed internal quality (new lines seem to have addressed this)

Real Sword
Pros
-easily the best bodywork of any aeg
-fantastic internal quality
-good stock performance
-did I mention the bodywork is absolutely solid?

Cons
-very limited range of models
-require a little more knowledge to upgrade effectively

G&G
Pros
-cheap and very good value for money
-decent bodywork
-good stock performance
-wide range of models
-higher tier guns have loads of nice features

Cons
-very weak gearbox shells
-lots of components are slightly off-spec so upgrading certain things is difficult

Ares
Pros
-some of the best bodywork on the market
-a wide range of unique models to choose from
-packed with features

Cons
-possibly the worst designed gearboxes on the planet
-almost everything is proprietary
-weak internal components
-difficult to upgrade (but it is possible)
-should really be classed as collector’s pieces

Systema
Pros
-excellent bodywork
-‘Real Steel’ compatible for many parts
-good stock performance
-chock full of features
-fantastic stock trigger response

Cons
-mind blowingly expensive
-terrible value for money
-a history of releasing broken components in ‘top tier’ guns
-many models require upgrading before they are usable (mostly older ones)

ICS
Pros
-thoroughly average in every way

Cons
-thoroughly average in every way

Those are the main brands that are generally considered to be good quality in the airsoft world.
I’m sure I’ll think of more that could do with a mention so I’ll update this if needed.
Let me know if you have any questions or comments.