I'm going to come out and say it. OMF is my favorite fighting game.
Now I'm not saying it's the
best fighting game but I was addicted to it when I was a kid and I still find it fun as hell today. It has a great atmosphere and it's deceptively as complex as it is simple. The robot designs are goofy but that's part of the charm.
The Tournament Mode is great and is possibly one of the first custom character features in a fighting game ever. The arena hazards were cool, the "destruction" fatalities are awesome (especially Shadow's, holy shit) and to me it all felt like a better Mortal Kombat but with robots.
Maybe the nostalgia goggles are stuck to my head with Gorilla Glue but I don't care.
There's an open source project going on too:
http://www.openomf.org/