Spectre question

I have a free gem slot so I figured I could just have a couple spectre's

Anybody know which are best for a caster?

Most important is that they stay with me when I log out and back in :)

There was a change to spectres this league.

"When Spectres die they now transform into a floating Orb Spectre that cannot be targeted and cannot use skills. When you enter an area the Orb Spectre will automatically transform back into the original Spectre. Orb Spectres can still be overridden when Raising new Spectres."

If your build is not a minion build, the tricky thing is keeping them alive to be useful.

It depends on your build, there would certainly be good ones for you, but if they are dying quickly it is just not very useful. You also probably need 2 slots free so you can have a skill like desecrate that spawns corpse if you are needing to res them often, though this may be is less necessary with the new change, if they now are automatically respawning on zone change.

There's a list of some of the useful ones here
https://www.poewiki.net/wiki/Raise_Spectre#Support_spectres

Corpse item specters are also good, but maybe not so useful if you can't keep them alive.
https://www.poewiki.net/wiki/Corpse_item