It's hard for me to make it a top 5 ranking .. what I play depends too much on mood and other players .. some games are just not mp-able, other only playable that way
SE4 (o.c.)
Stars! (MP/email only)
SteelPanthersWorldatWar (it's free)
MOO2
EF2000
TTDX (with J.Drexlers patch)
BOTF (with my own mod)
RedStormRising
Harpoon2AE
.. other I'll touch only once or twice a year
A.