Free $20 March Madness Bet: How to play it?

Quote: TIMSPEED

...free $20 sports bets...

I assume this is in the US. In the UK you can get "free" bets, but usually they are bets where you keep the winnings but not the stake (so worth checking the rules for yours). In this case, all other things being equal, then long shots are the best option.
If it does include the stake, then if you have multiple bets you may prefer to bet both sides to ensure a profit.