Enchanter Spell Gems

[Killval]

So what is the best spell to have specifically in spell gem slot #1?? I have only a lowbie level 25 Enchanter but I love playing him a lot, my favorite caster class, with a Necromancer following closely behind, but currently have been using Tash in spell gem slot #1, although I'm debating if Root might be better? Why spell gem slot #1 is so important is because with a GCD reset item such as the Rod of Insidious Glamour you can have spell gem slot #1 always available pretty much, no cooldown. So what would be the most advantageous? Or would it mostly vary on whether or not you are soloing, duoing, or grouping?