5 since 1999?
Without googling I'm not sure if your moniker (the late Mr Vialli) was pre 1999, so I'm guessing:
1/ My all time favourite non-City player Jimmy Floyd Hasselbank
2/ Zola
3/ Drogba
4/Shevchenko? (world class, but not necessarily at Chelsea)
5/ Gudjohnsen (or was he classed as midfield?)
5/ Torres (see Shevchenko)
6/ Weah (see Shevchenko)
I'm sure I've missed an obvious one post Osgood and Dixon
PS I've attended almost 1500 City games, but the first goal I ever witnessed was scored by Keith Weller in 1971