Deadly Icicle (Lv5) - Max it, no exceptions. Illu's main damaging skill.
Ancient Call (Lv10) - I put 20 points maximum into it, but usually leave it at 1. It's decent when soloing and useful to lure single mobs, but you don't need HP for those scenarios. In a dungeon pets tend to cause more problems than help. If you're aware that you can right click to remove the pet buff and desummon it you can alleviate this issue. Could be maxed later on or if you don't want Mind Control.
Stimulation (Lv15) - An Illu's most useful party buff. I always max it but it's up to you. I rarely keep Stim up while soloing, so if for some reason you never plan on playing Illu in a party you could ignore it. Chances are you'll want this maxed.
Mind Surge (Lv20) - Another one of the Illu's useful party buffs. Doesn't need to be maxed but should be high enough to take care of your group members mana needs. Also be aware that you will often need to move closer to the tank so they get the effect as well.
Mind Control (Lv25) - All or nothing skill. Only works on mobs that are a lower level than the skill level. If you don't max it out it's not useful against anything you'd need it for. If you do max it out then you need your Illu class to be very close to your level otherwise it's still not going to be high enough to help. If you are capable of using it on enemies it is very very useful in certain situations.
Hypnotic Eye (Lv30) - A very useful skill to survive bad pulls or mistakes, however it works fine at Lv1. Higher levels only increase the damage of the next hit (and rarely will you be using this for increased damage) so I say ignore it until you have plenty of extra points.
Psionic Focus (Lv40) - Decent when soloing. I can't recall off hand if it can be used in combat (Wiki says it can't). If it can't be used in combat then it's basically the same as Sneak. Good when soloing to help crit now and then, but useless in a dungeon since the tank will pull first and force you out of invis.
Ineffective Magic (Lv50) - Never used it personally but I can't see this being good for anything but PvP for the reflect. Rarely do non-boss mobs use magic skills and even when they do it's usually too strong to make straight -dmg effective. If it's an extremely high increase per level it could be worthwhile.
Aerolite (Lv60) - Again I've never used it, but given that it's an Illu's only AoE + a Damaging skill, it's probably worth maxing out. By this point you should have a pretty good idea of whether you want to level other skills or not and decide for yourself though.