Gungnir is a solid default, as it is useful for everyone. Some characters/specs have better options though, for instance:
Physical melee: savage axe of ares
ranged energy: cosmic control rod
ranged mental: warlock eye
pet-heavy build: neptune trident
But yeah, if you aren't sure just get a gungnir. You an equip it on any char at any level, but if you want to unbind it for another char it resets it to 0 xp.
I would adjust this a bit:
Melee based characters = excalibur (be careful on what is/isn't tagged melee)
non-melee based physical characters = savage axe of gungnir (the difference is usually very small)
Gungnir = hybrid damage characters (Thor)
Other:
M'Kraan will likely be best for Ironman and no one else after it's 3rd? 4th? rework.
Shield of Perseus for "ultimate tank" characters. Not "worth it" for current content, even raids atm but if you don't care, the proc is pretty awesome.
Apollo's Bow is actually not bad for a lot of ranged characters that don't have much natural attack-speed in their kit (and benefit from it). The proc damage is underrated a bit but ultimately will be out-scaled by gear.
Nornstones sucks. Wait for rework.
Nullifer = PvP only basically where burst/spike seem to matter more (or so I'm told, I've never PvP'ed)
Think that's all of them.