I say Rabbid Luigi is the most underrated character. I admit that I didn't use him much when I first played the game.
Movement-wise Rabbid Luigi has incredible base movement and glide time, tied with Princess Peach. Exhaust Dash is great because it affects both of Rabbid Luigi's Dashes.
His weapon can help so much when fighting lots of clustered weak enemies, Darkmess eyes, barrels etc. especially when he has 7 bounces to his Discruptor. His damage is rather weak however and not very effective against strong enemies/bosses but the "Vulnerable" upgrade helps a bit.
Exhaust may be ignored by players because it doesn't fully protect your heroes, but it has long range and a fast cooldown to make up for it. 70% weaken and the "Vulnerable" upgrade make it a helpful support ability and can help heroes who get close to the action (Edge and Rabbid Mario).
Rabbid Luigi excels at hitting so many things and weakening strong enemies. He's super helpful against the Giant Depleter in my opinion. Unfortunately, he is terrible when up against Magikoopas because they power themselves up, removing the weaken in the process. Midnite also has that ability.
Rabbid Luigi has his ups and downs, but in the end, he's an all around character with good movement, HP, group damage and support.