From a Liverpool point of view, I definitely agree on Martin Kelly, gets the number one underrated player for me. I trained him in every way possible, got him playing first team footy in three different loan spells, and still couldn't get him above 12 in some of the major attributes needed.
David Hoilett underrated? He's in my current save, I'm playing him as support at right wing for Derby County with team instructions down for him as playmaker.
Stats:
2011 - 31 appearances 3 goals, 13 assists
2012 - 30 appearances 3 goals, 13 assists
2013 - 34 appearances 7 goals, 21 assists - season not quite finished yet.
If you think he's underrated then fair enough! I'm happy to have him but irl if he achieves those stats in the next three seasons, he's a **** of a player