what're your priorities?
if you like the statement "accelerated so fast I could barely keep my hands on the steering wheel..." then get the C32. if you want lots of great performance with more road-feel and a nicer interior, get the S4. if you want a power-cruiser, get the 545. the others i wouldn't even consider.
looks, power, and prestige, the C32 wins.
the 545 is really hit or miss--the active steering is awesome but the looks are very questionable and performance is amply powered, but somehow unsporty--it's a boat. if you like cruising and comfort with a big engine, get this car.
the S4...are you talking 2004? if so, i would choose this without a doubt, with the C32 coming in second. i mean the S4 is what every member of this board must (in some way) wish he/she had purchased (the old version i'm not referring to). people try to mod their E46's and still can't even match the stock performance (in any category) of the S4, let alone a modded one. everyone admits the interior is better, so the only thing missing is possibly exterior looks (only by a slight margin) and the name (BMW is onstentatious)
i can't even believe you're considering a ZHP in this line-up. it loses in every single category.
the G35 (sedan)'s only redeeming quality is that it's so ugly.
with price as no consideration, S4, then C32, then 545. otherwise, it gets a bit tougher.