I have a lot of different favorite cars in GTA IV, and what I pick really varies depending on the current situation. However, here are the ones that I always try to grab if I get the opportunity:
Intruder
Oracle
Admiral
Blista Compact
Sultan RS
I like my four doors for some reason, even though the ones I have outlined are not the most high-performance. The Intruder and the Oracle look very nice in black - almost like the kind of car that you'd see in The Transporter films or something. Plus, there's something really quite exciting about using a lower-performance car in Free Mode when someone has decided to hunt you down for whatever reasons they have. It's just quite hilarious ducking and weaving through the traffic on the wrong side of the road and watching them stumble with them. Or just hammering it up the road normally is always fun too