I naturally find myself trying to uair combo with Luigi, it comes from playing Mario so much
Uair doesn't combo very well, but it's easier if you're uair comboing them through platforms. Like jump, uair, uair, land on above platform, continue combo. Or short hop uair, fall down to next platform, uair, continue combo.
Uair works best at very low percentages, like 0-20. Might vary depending on who you're fighting against.
I'm still experimenting with this. Oh, a couple others I just remembered. They're all very similar but they work well. I'm going by memory here, let me know if some aren't right.
At low percentages:
SH uair -> nair -> utilt(s) ->continue combo
SH fair (helps if you have forward momentum) -> uair -> utilt(s) ->continue combo
SHFFL uair -> utilt(s) -> continue combo
Ledgehopped uair -> nair -> utilt(s) -> continue combo
Ledgehopped fair -> uair -> utilt(s) -> continue combo