It really depends on what stage u are, and what your focus is.
Istina:
Pros: Nice for pvp, High mdef. (crit rate is only nice if u got general bad setup with low crit rate)
const. No real usefull bonus beside Mdef, again crit only nice if u have bad gear. overpriced.
AQ:
Pros: 15% crit dmg(this matter alot i pve makes tyrr really powerfull :D) great vs all but mages in oly. u need the dmg vs healers. Cheap
cons: Low mdef, bad in open pvp, party pvp and sieges.
Oly Ring:
Same as anqtueen, 60 days sorta free AQ, cant be enchanted though.
Baium: 15% crit dmg(same as above), some ok boost for atk and cast spd. Cheap
cons: same as above.
Octavis
pro: High Mdef. 15% crit dmg, generaly must have.
cons: REALLY pricy, hard to come by.
Those are your options on low budget. the endgame uber option.
Earth wyrm ring and Tauti Ring.
If u have a desent crit rate, 450+ i surgest a combination of OE Seraph jwl and AQ+Baium untill octavis earth or tauti is a option. I would never waste 1 adena on istina couse its just not worth it.
For other jewelry:
Earrings:
(Blessed) Antharas.
Blessed Zaken
Istina.
Necklace:
Istina
Octavis
Order of purchase.
AQ+Baium first to improve pve qualities.
Then any of the necklace earring.
Then upgrade rings.