My mains so far have been Marth,Peach,Captain Falcon and Fox. I'll have examples below.....
Marth- I have a huge amount of experience with marth because of melee so it seemed natural to play him in P:M, I have always enjoyed his strong aerial game and his dash dance is huge compared to others imo. His ability to punish people while in the air is amazing and is just too great <3.
Peach- I recently discovered peach's power in melee about a year ago now and mained for about 8 months and with P:M here now i was able to bring her destructive Down Smash back into action and her great edge guarding ability
Captain Falcon - two words " The Knee "
Fox - I bandwagoned extremely hard when fox was getting abused in tournaments and destroying everyone in melee * and still is in P:M * I enjoy his speed and ability to execute easily with his Up air :D
Most of my playtime has been taken up by these four characters but i plan to add lucas to mains after i recently discovered his ability to combo and juggle extremely well :D