Well this is always a tough one for me, to pick a favorite of somany of the greatest hyms ever written,
Well lets see,

I guess my top 5 right now would be;
1. It is well with my soul
2. Victory in Jesus (Proabably one of the main hyms that helped lead me to the Lord)
3. Living For Jesus
4. A mighty Fortress is our God
"A mighty fortress is our God, a bulwark never failing, our helper he amid the flood, of mortal ills prevailing, For still our ancient foe, doth seem to work us woe, his craft and power are great, and armed with cruel hate One earth is not his Equall.
Did we in our own strength confiede, our striving would be losing, Were not the right man on our side the man of God's own choosing. Dust ask who that my be? Christ Jesus it is he, Lord Sabbaoth is his name, from age to age the same, he must win the battle.
That word above all earthly powers, no thanks to them, abideth; the Spirit and the gifts are ours, thru him who with us sideth. Let goods and kindred go, this mortal life also; the body they may kill (I'm not possitive on this, but I willing to bet that either Luther wrote this shortly after the diet of Worms, or at leased he had it in mind when we wrote this.); God's truth abideth still; his kingdom is forever.
5. And Can it be.