Alan:
Magneto - Disruptor (fast, beats other projectiles)
X-23 - Ankle Slice (hits low, otgs)
Wesker - Samurai Edge (also hits low, otgs)
Assist types vary by character. Not everyone has AA or launch etc.
Edit: or were you asking for assists that benefit them, rather than each char's best assist?
For Mag, I imagine anything that hits hi/low (burning kick/demon flip/aforementioned lows) would suit him.
Don't know about X-23... someone w/ a projectile?
Wesker benefits from assists that enhance his teleport game (Sentinel drones works wonders, for example).