ACRyan has most of the features that you are looking for in media player.
Your requirements as mentioned by yourself (with ACRyan's feature match)
1. Play almost everything - Yes
2. Easy to Use - May be or may be not (more of a personal choice)
3. Ability to play over n/w and wifi also - Yes
4. Want to connect a 5.1 channel system to it - Yes
5, HDD slot a must as i want to avoid plugging in external USB HDD everytime - Yes
6. Obviously firmware upgradable - Yes
7. Cost effective..(plz consider as per US prices as i intend to get through that route)-Yes
8. Access to online medias like youtube etc and photo sharing also directly through the player - Yes/No
So, go ahead get ACRyan PlayonHD