I also vote for the Rekross' shield. It is a wickedly useful find if you happen to get one around lvl 15-25. Once (quite some time ago, before the english version was launched) I had it spawn on Rusty and I couldn't even begin to imagine how easy the game suddenly became. Another time I had it for free in Montero shafts, but I didn't know of its existence
Other Items I think that are great are:
Black Helmet for the BK tactic
Student Belt - free class runes every now and again, even if you haven't equipped it. The game can spawn up to 2 of them!
Martyr's Boots - +4 Intellect, +1000 Leadership, -1 Speed - great for a mage who plays with lvl 5 units (especially dragons), because for them the -1 speed is a mere pittacks.