It depends: Do you run Legacy, TMCC, DCS, or conventional? Also, is the rest of your rolling stock and scenery scale-sized, or "traditional" O gauge. There are several good choices- choose the version that best complements the trains and layout you already have!
That being said... I know you have some LionChief Plus locos. I was really hoping Lionel would reissue it's "semi-scale" K4 (check out 6-38089 for example) as a new addition to the LionChief Plus line. @Dave Olson Maybe in 2020 they will!