The first thing is which period of the Italian Wars you want to. If it is the very early period then late 15th C figures like Mirliton would be a possible solution. For the 1520s (my own choice for my French Army), for the best results, I think you need a mix and match approach and to be willing to do some minor conversions. In particular, I think the key to mixing different manufacturers successfully is to replace spears, pikes and lances with wire (and maybe heavy weapons).
I am using a mix of old and current Minifigs, Frei Korps, Asgard/Table Top Games/Altuos, Essex and now Khurasan.
I have a unit of Essex Gendarmes. These are nicely moulded with a variety of styles and poses but lots of the horses have a rather head-down depressed pose and a lot of them are not really right for French or Italian. I also had to add or extend their skirts. The way they hold their lances looks a bit odd and the lower part of the lances, below the hand is too long. I replaced all these with wire and Milliput.
My other Gendarmes and command units are a mix of old and new Minifigs and Altuos. Again, I replaced the lances and made some simple modifications to skirts and horse barding.
The Landsknechts are another mix: old and new Minifigs, Altuos, Frei Korps and one Black Hat (middle standard bearer).
My Aventurier Arquebusiers are Old Glory with a converted Essex sword and buckler man as an officer. As standard they looked too much like Landsknechts for me and I have modelled new hats and helmets for some of them to give a more mixed look.
I will be doing some Swiss next which will be a mix of Khurasan and Minifigs (a good match for height and build).
Although I don't have any photos, my artillery are Minifigs and Altuos. For Stradiots, I used the same mix and old Minifigs for mounted cross bow but if I did them again, I would use Mirliton from their Condotiere range.
madaxman.com has a useful and very comprehensive listing of manufacturers and many comparison photos.