I would say sign up for all that are legal in your state.Most offer risk free sign up bonuses and if you play your cards right you can end up making money off using the funds from the sign up bonus .Also the DFS apps like Prizepicks,Underdog etc... have sign up offers as well. Just be sure and read the fine print on the bonus, how long you have to use it etc...
Most people start out with Draftkings and FanDuel. In addition to the sign up bonuses, make sure to check the promotions for DK and FD daily. For baseball right now DK has a lead by 2 special. No matter what team you bet on if they ever have a 2 run lead you win, regardless of the final outcome.
For example Phily as a -200 favorite yesterday beat the Rockies by 8-4. So naturally bets on the Phils cashed. But CO was a +200 dog and at one point CO led 3-1. If you bet CO using the promotion you were able to cash a winner in the bottom of the 3rd, despite the fact that they lost the game by 4 runs...