He's never been outright bad except for 64.
Melee he's a mid-tier, but Eddy Mexico beat Mang0, so he can definitely get the job done.
Brawl he's also a mid-tier, but being a mid tier-in Brawl is a lot harder than it is in Melee because of how dominant the high tiers are.
I can't speak for P+, but PM 3.6 Luigi was also a mid tier, in similar standings with Melee.
Smash 3DS (aka Early Smash 4) Luigi was really good. His downthrow combos were incredibly strong and he had lots of cheese with Cyclone.
When Smash Wii U came out, Luigi was also really strong, but late 2015 his downthrow combos were nerfed. Despite this setback, he still remained a solid high-tier pick.
Smash Ultimate, he's been a solid pick since day 1. Ultimate upped the power level of the whole cast; Luigi's combo game became insane, with zero-to-deaths off his grab galore. He also got a great zair for walling people out and a frame 1 combo breaker in his Down B.
Pound for pound, I'd say Smash 4 Luigi is the best he's ever been, but that's relative to the rest of the cast.
Though, a few weeks ago a Japanese Luigi player named Raru won a major tournament with him in Ultimate. It's the very first time a Luigi has won a major in any Smash game, so speaking purely based on results, you could argue Ultimate is the best he's ever been, and I wouldn't argue.