I started with AudioTechnica ANC-7b.
The noise cancelling was an eye opener.
But it broke. Cheap plastic structure.
Next was ATH-M50(which I gave away recently)
It isolates well and sounds pretty good.
But the earpads suck and gets stiff and flat pretty soon. Need to replace the pads frequently and becomes expensive over time. 3rd party pads sound horrible.
Since then.... I still bought many headphones like Sennheiser HD598C, Monoprice M1060c, Fostex T-X0, Fostex TH-X00 Ebony...
But the best result in terms of isolation is Shure SE846.
I would rotate among these and many other IEMs.
It really depends on what you are looking for.
Do not go for semi-open or Open back headphones if there are people sitting close to you.
Another thing you need to consider is... in case of office announcements including emergency evacuation broadcasts.... you might not be aware of them and some companies prohibit usage of isolating IEMs or Headphones.