In Halo CE, it's the Needler. Yes it may easily be the worst weapon, but one can't deny that skewering en enemy full of needles which proceed to blow it up (sometimes twice because each explosion takes seven needles) is fun. Plus, it's quite low-maintenance because there's many enemies that use it and, unlike the plasma guns, you don't have to keep on switching them because it uses a normal magazine instead of batteries.
For Fallout: New Vegas, I honestly feel like the Trail Carbine is the best-looking lever-action yet it's grossly overshadowed due to the Brush Gun above it and the Cowboy Repeater below in spite of being a great middle ground that can even be modded into a decent sniper rifle. I also think that the Single Shotgun is better than the Caravan Shotgun due to being more accurate and having proper sights. Finally, I often see people complain about the Automatic Rifle's poor accuracy, but I really like it both because I'm a sucker for slow-firing automatic guns and because it still hits rather hard.
In Terratech, one thing I was surprised is to see people on the subreddit rib Venture stuff, but I really liked it because my favorite builds are prop planes and similar stuff, plus the missiles are easily the best at keeping up with the fast plane you can easily come up with.