Best: Allows for historical imports from the Lahman database so you can compare different teams and players from different eras. You can experience the Dead Ball Era or the Steriod Era - either way. You can pitch Warren Spahn against Babe Ruth - the 1927 Yankees against the 1998 Yankees (with a bit of import work). Tremendous value and flexibility for the historical baseball fan - as for those who prefer a fictional approach. Admittedly you won't get stats-based type results but most of the time the engine gives you "reasonable" results that feel completely realistic.
Worst: The in-game features. Actually managing an individual game is not very spectacular. Pitcher fatigue is not as sophisticated as it should be as far as managing an entire staff through the course of a season. Also, handling historical league expansion is a bit cumbersome.
Overall, OOTP is the best baseball sim value on the market today. Hopefully, you guys can make it even better!
Thanks for asking.