That's a toughie as there are loads of good ones to choose from.
For me its a toss up between Yesterday's Enterprise and Borg Are Everywhere. I like the bleak, hopeless situations both are in, both of which will test the character of even the most hardened of Starfleet officer (Riker had obviously cracked under the strain).
Commander Austin Harris, First Officer, Deep Space Nine (by FltCpt. Bossco)
8.01 - Darkest Before Dawn
(Chapter 8 added, 12/09/2015)