ort/ral/thul if you need resists
rubies for hp
topazes for mf
ums if you can afford them
poison facets if you're poison
Not to sound mean... but look at what you're missing and socket to fix that, this is a rather basic sort of question. I made a tal's sorc last season, she didn't have any poison resists but was stacked everywhere else, so I put tal in tal's armour+ss, there is no best thing to put in any piece.