Started my journey to IEMs with Etymotics c. 20 years ago. Have gone through dozens and dozens of IEMs since and have a huge collection of (mostly sucky) tips of all sorts of shape/dimensions/materials in my cupboard.
My take:
The absolute two best tips for isolation (most amount of dB damping AND widest frequency dampening) are:
Comply audio Professional P-series Foam Ear -tips
https://www.complyfoam.com/products/professional-p-series
Westone Audio Performance Foam Eeartips 14mm (longest)
https://www.westoneaudio.eu/collect...ance-foam-eartips-14mm?variant=41198102478980
Both are deep and wide enough and out of a dense enough material.
However, both of those limit you to IEMs nozzle/tip size of (tip inner bore hole) size of 2-3mm, so basically Westone/Shure IEM sized models (out of the Chinese makers these used to be models like AudioSense T260Pro, NiceHCK DT300 Pro <- not sure they are available anymore).
Of course, if you get a custom mold WITH additional foam tips, you might get same/similar isolation, but all current wide-nozzle IEMs are much worse in passive isolation compared to the above models. None of the silicone/rubber flange tips come even close (well maybe the original ETY if I used the thickest ones and inserted them so deep that they caused serious pain in my ear canal, which is not very practical).
None of the other tips come even close. Both above foam tips are deep inserting, very long and come in at least a few diameter sizes. The thicker models your ears can tolerate, the better isolation you will get.
The problem is finding modern good quality IEMs (sans the few Westone/Shure models, which have been mostly surpassed in performance/price ratio), that still have that magical 2-3 mm nozzle diameter.
Any suggestions for IEMs with 2-3mm nozzle size that are still available for sale (just checked, the basic and upper range AudioSense and NiceHCK models are not being sold anymore).