Luigi: Easy and devastating combos, but the playstyle is braindead and kinda boring. Against a great player, struggles at recovering.
Mario: Fast and aggresive, he's easy to learn and the perfect character for getting better at reading opponents. Amazing combo game.
Dr. Mario: Kinda the opposite of Mario, slow and deffensive playstyle. Shares Mario's framedata but he's slower. Hits harder and kills earlier. Combos harder to pull off.
I really think that Luigi > Mario > Dr. Mario, but it depends on your playstyle. I main Dr. Mario and I do pretty well with him, and I have NO IDEA of playing Mario, I hate him. I'm super deffensive and doc is the perfect character for me.
Also Dthrow to Fair is a true combo at some percentages on most of the Top/High tiers like MK, Pikachu or even Sheik. They die at 70% and can't even airdodge.