I believe Block is the most useful skill because:
a) it's available to almost any player
b) it will save you a lot of rerolls (and frustration)
c) it makes one dice blocks far more reliable
d) it can be very effective for knocking down opponents
e) it makes your guy harder to knock down



When choosing skills for your players I would suggest that you look at http://fumbbl.com/skills.php to see what other coaches select as their first skills for their players. The majority of the coaches on this site are quite good, and generally have a pretty good idea of how to play the game well. The skills that rank highly (for your team's race) are generally good skills for you to choose.
You will notice that block ranks very highly in most lists. There are of course exceptions: Dwarves and Norse already have block on most players, and goblins, halflings, skinks and big guys can't access block unless they roll a double on their skill roll.