I like Extended Rollercoasters at the moment.
They have alot of unique track elements (able to run upside down, different shapes for bumps, unqiue looking inversions etc) and also different car styles; Basic, Twister, Floorless, Hypertwist, Thin Hypertwist, stand up for example.
It just allows you to be very creative with your designs.
Extended coasters allows you to creat mind-bending underground sections where you cannot keep track of which way is up anymore.
For an inverted coaster type, the classive Inverted Rollercoaster is my favorate. It doesn't have as many different looking inversions as other inverted types, but you can't go wrong with the Large Verticle Loops and two theme car options.