The best I’ve found for me are IEMs with narrow nozzles, that can fit Shure tips, such as Etymotic and Westone. I use the triple flange Shure tips, but cut off the third flange. It makes them quite a bit more comfortable, helps them not suppress the highs as much, and isolates just about as well, subjectively, as Etymotics with their triple flange tips. This is the combo I use currently for drumming live (in a fully enclosed cage/room...so VERY loud).
The absolute best isolation is by using the Shure multi flange tips, then directly placing Dekoni Bulletz Mercury 4.9mm tips over the stem/base of the Shure tips. It creates a multi-flange hybrid silicone and foam tip, and no modding is needed. I don’t use these tips for drumming, b/c as loud as it is, these actually block out too much of the direct sound from the drums.
...and on Etymotic IEMs
Just compress/roll the foam, insert, then ....no sound, until you press play.