I have no idea what you're talking about with the SV. Mine sticks to the road like glue, I can floor it as much as I like and there will be only minor understeer (with full throttle out of a turn, mind you).
I love it to death, it was like my 3rd car purchased (up to 106 in my garage) and it is still my favourite. And no, no assists, I've only got ABS on 1.
As for "the hardest car to drive", it depends what you mean by "hardest"... Overall? Because you just want to see if you can drive it? Or hardest car that actually has a use? If it's the use one, I'd say the Ferrari 512BB, because it is still good and can be used to win the Italian/Ferrari events, except it requires a lot of effort to keep under control.
Overall... Not sure, the NASCARs on non-oval tracks are a bit of a pain, they turn pretty wide and still have massive oversteer. That Top Gear Lotus was horrible too. But, a lot of the muscle cars are HORRIBLE just because they feel like a barge being driven by a drunken Irishman.