If the fork choice has to define the bike, then I’d go for the Five.
That aside, the choice depends more on your proportions and intended riding though.
I have a 2010 Five with 44RC3 Ti forks and love it. It hasn’t felt out of its depth yet in Lakes, Calderdale, etc, which is where it gets most of its use.
I’m 6’2″ with a 34″ inside leg and the 20″ Five is perfect for me with a 50mm stem as I’m long legs/shorter body. It’s far more manouverable in tight techy stuff than the Commencal Meta 5 that I rode previously.
Haven’t tried the Mega but I’d not build one with less than a zochi 55/Fox 36/RS Lyric.
As other have said, I’d think of the Mega as more of an Alpine 160 equivalent.