Maradona to Napoli would have to be up near the top. Henrik Larsson played a big role in turning around Celtic's fortunes in the late 90s. Rangers had won 8 league titles in a row (Celtic had nearly gone bankrupt in the eraly 90s too iirc) before Larsson joined Celtic and won the league in his first season.
Vds, ronaldo (hard to say, he's been with us sincr youth), ronaldo fat one at inter, ronaldo at madrid and figo at madrid (in a way they define no transfer is impossible)
Tevez - came to United and immediately won the PL and CL. fecked off to Citeh and they began winning trophies, off to Juventus and led them in scoring and to the Serie A title. Amazing catalyst for Boca as well.
Not so much for the national side except for the olympics.