1. I really like some of the Statcast based metrics which have become available recently. Both xWOBA and xERA are very handy tools to use for evaluating player performance. These tools are able to remove defence from the equation similar to metrics like FIP, but also incorporate batted ball data into the final result. One good example of the utility of these tools is apparent with a player like Marco Estrada. During his Blue Jays tenure he ran low ERA's, but high FIP values. He was able to induce a lot of lazy fly balls and popups, and a metric like xERA very closely mirrors his ERA results, working as a means to validate the ERA values he garnered as being legitimate and not a fluke.
XWOBA is useful for attempting to determine which players have been lucky and conversely those who have been unlucky. Using BABIP alone isn't sufficient as it doesn't directly take quality of contact into account. Players like Santiago Espinal and Reese McGuire have both produced much higher actual WOBA results compared to xWOBA, which suggests they may have been a bit lucky to have the success they have enjoyed up to this point. At the opposite end of the spectrum Rowdy Tellez and Danny Jansen have both dramatically underperformed with much lower actual WOBA compared to xWOBA. This may suggest they have experienced quite a bit of bad luck.
XERA and xWOBA can be found at https://baseballsavant.mlb.com/ and fangraphs as well. Baseball savant contains a treasure trove of information. You can find detailed breakdowns for individual players for how they compare in various batted ball metrics against the rest of baseball, player sprint speeds, defensive information incorporating outs above average as well as other information.
https://www.mlb.com/glossary/statcast/expected-woba
https://www.mlb.com/glossary/statcast/expected-era
2. My favorite website for calculating player values is https://www.baseballtradevalues.com/. This site breaks down every MLB player and each team's notable prospects into a dollar value. Salary and years of control are included in the final values. You can use this site to work out potential trade scenarios which is pretty fun to do.
3. For prospect values I like https://www.mlb.com/pipeline. This site has a ton of prospect lists, and gives full scouting grades to each team's top 30 prospects. Compared to Fangraphs the grades are definitely more on the generous side, but you get detailed writeups as well as the grades for each prospect's tools.