I have played dota and lol for about 2 years and joined HotS in early alpha and i am a master illidan and this is the build i usualy use

LvL1 (shadow shield) gives early survivability against spells that arent blocked by e and later gets quite big

LvL4 (thirsting blade) more sustain so you can stay in fights longer

LvL7 (follow through) illidans cooldowns are really short especialy with his passive so this is very effective

LvL10 either ult works but i usaly choose the hunt, Metamorph is best used in maps that make lots of teamfights like cursed hollow and the hunt when there are lots of small skirmishes happening like blackhearts bay

Lvl14 (sixth sense) this makes illidan be able to take much more damage and let him live longer

LvL17 (secound sweep) this helps his chasing, running away, body blocking and keeping the bonus attack damage up which is all very good so its a versatile talent

LvL 20 always upgrade ult if you get metamorph but if you get the hunt it depends on the game, are there always fights on the other side of the map? If so upgrade it otherwise i get fury of the storm for easier merc camps and wave clear and little more damage in Team fights

Feel free to comment if you like it or if you think it needs improvement