Considering that I can choose any seat on any seat row on a Boeing 777-300ER for an international flight, I have most of 400 seat places to choose from the forefront to the last row of this airplane. What are the best options if I bought an economy class ticket, traveling in a group of 4 people and if I want two windows? Are there any advantages in choosing the last places instead of the front ones?
This is a very difficult question to answer in general, because different airlines have different seating configurations, seat pitches, etc. The best thing to do is to check out sites dedicated to airline seating plans, such as:
All of these sites are built on a user-submitted database + aircraft seat plan data on what the best seats in an airplane are, according to aircraft type, passenger class, short/long haul sectors etc. I see that you've already checked SeatGuru; beyond that, you may also want to check one of the others ones to see if SeatGuru's missed out anything. Usually, they're pretty standard.
For economy class, unless you have extra legroom or reduced recline or proximity to galleys / toilets, there literally isn't much that distinguishes one seat from another. After that, it all boils down to personal choices. I usually like aisle seats for extra legroom. The back of an airplane will experience more G-force during ascents, so some people prefer seats towards the front of the aircraft. And so on.
Everyone likes extra legroom, but as an economy class passenger even if you check-in online, chances are you won't get those seats on long haul flights because many airlines open up online check-in for those in higher frequent flyer tiers earlier. This gets easier to snag if you're a solo traveller because it's much more likely to find singleton seats later in the checkin process.
When travelling with four people, on your flight the middle row will allow you all to sit together. If you want two window seats, then it's going to be quite awkward if you plan to talk during the journey. My advice? It's a long haul flight. People around you won't like people chatting. Tune in to the in-flight entertainment system, read a book, or just zone out on any seat you get.
Interestingly, earlier this year a 'study' by Skyscanner came up with seat 6A as the best seat on the plane. They polled visitors to their website:
It determined 6A was regarded as the best seat – putting aside planes that offer first or business-class travel.
However, they also notice that the best seat inevitably comes down to personal choice - some like the window, some like aisles. Personally I have no sense of smell so prefer the back of the plane, on the aisle, so that I can get up and stretch and wander.