Well, there are a lot of Ford crown victoria's, dodge chargers, chevrolet impala's and tahoe's out there, both slicktops (yes, that's without a lightbar on top) and marked ones. The thing is, we can't decide what you like. It's up to you to decide what models and/or skins you like. I'd suggest you take a look at GTApolicemods and the donwload section on this site and take your pick.
Another choice you're going to have to make is whether or not to go with ELS. You have to realise that replacing all the police cars with ELS ones will put a lot of strain on your system and will surely cause some random crashes. Try to keep ELS for cars that don't spawn that much, like FBI and NOOSE and keep police1 and police2 non-ELS.
However, that's just some advice, you're free to do whatever you like of course.
If you have any questions after you made your choice, feel free to ask.