In general you will get this answer because the 550i has a V8 engine and the 535i has a turbo. The 528i has a straight-six. A more straight-forward design with less moving parts increases reliability. However, any of these cars can and will last a long time if properly maintained.
i had a E34 525i and by far the best car i ever owned.. Never had any trouble with it but i always done an oil and filter service every 5/6 thousand and when i bought it, it came with a full bmw service history. Had it for 4 years..
I believe your 525 is basically same as 528i or 528i came from 525. That is probably why 528i is considered most reliable BMW series. Perhaps, BMW learned from 525 to produce the later.
I am glad to hear you have had good luck with 525. I hope it last over 200,000 miles! Look under car and under hood. You really can tell it is well designed and well built.