It really depends on the matchups and your teammates. Octarine is usually my first or 2nd item, unless I need Euls, forcestaff or blink.

I get Euls when I need to dispel silences or to cancel channeling spells (your 6s cd may be insufficient) or when I'm up against a Lina or Invoker with Euls.

Blink or forcestaff? I get blink when there is an initiator and it is crucial for me to immediately follow up. For example paired with Void, it is absolutely crucial I get blink so I can either blink behind the backlines to burst the supports or blink beside the chrono to ult. If not, most of the time I get forcestaff for escape and for extra mana/health and utility.

My item progression goes like this: Arcane boots > aether lens > blink/forcestaff > euls if needed > octarine > bots > refresher

If I need damage early or there are multiple sources of magic damage, veil would be good.

Knowing when to engage and disengage is absolutely key. You almost never go in first and you bail at any sign that they can close gap. My nightmare is to play against CK.

I only pick Zeus when I need heavily magical damage and when there are other frontliners or initiators. I particularly like to play it against PA and gank really early. Your rotation is almost certainly going to nab a kill.

Good luck.