Metal Gear Online was my favourite this gen. The community on the eu server was filled to the brim with cheaters and assholes, but when I first started in mid 2010 the community, especially the legit high levels in combat training were fun to train with. It sadly went to crap in 2011 though sadly.
Most of the maps were fun, I think a real time light system with moving shadows would have been amazing for some modes.
Customisation for characters was really in depth, and you had a variety of skills you could use, and level up further by using them.
The game modes were all fun for the most part, and some would only work on MGO. However most people insisted on playing everything as TDM, and would either group team kill or group kick you, even if you were the host, if they disagreed with your rules, instead of leaving themselves, before more often than not closing the match straight after.