Triarch Stalker
A word of warning to this page, if you don't like complicated as fuck models, you will rage.
Triarch Stalkers are giant Necron metal spiders. They are traditionally used for flanking maneuvers and sideswipe attacks, but can do a surprising amount of damage when deployed on the front lines. They can wield a fusion laser cannon that can either be focused into a medium range, melta-style cutting laser, or a wide, fanned out blast of boiling plasma. Their advanced targeting relays can bolster any nearby Necron units' accuracy programming.
The Triarch Stalker is a walker that ignores Crew Stunned/Shaken, has Move Through Cover, and possesses Quantum Shielding, giving it AV 13/13/11 until the first penetrating hit, all for 150 points. It carries the only Necron melta weapon, which it can alt-fire as a heavy flamer, and makes every unit that fires on its target twin-linked, for that extra dollop of "fuck you." It makes 3 attacks at S7 in close combat, and only the hidden power-fist (or equivalent) can really scare it. You can swap out it's Heavy 2 melta laser for a twin-linked Heavy Gauss Cannon (S9 AP2 36" Assault 1) or the Particle Shredder (S7 AP4 24" Large Blast).
While an effective unit, it's another example of "GW originality", drawing its inspiration from the Geth Colossus of Mass Effect.