The best:The Optima has been a reliable vehicle for me. I have put over 200k miles. I know that there have been lots of issues from others with the engine. I have done oil changes every 5k miles with full synthetic. I have only had to do minor repairs.
The worst:Burns oil which is a typical complaint about this vehicle.
The best:Great gas mileage, and for a 4cyl it has v6 power because of its high compression. Very fun to drive. Kia replaces the bad engine problem 100% on them.
The worst:Not reliable. Had an engine rebuilt or replaced. im not sure which one. But it was at 50,000 miles and now its almost 100,000 miles and has lots of glitter and metal flakes in oil. Probably about to die again. Engine replacement doesn't fix the bearing issue.