It's a difficult question to give a generalised answer to. I'm obviously not best placed to give a more definitive answer, but here's my opinion based on 5 days of zoo visits. The standard of Singaporean zoos is generally very high. Each of the 5 major collections (4 WRS parks and SEA aquarium) is of outstanding quality, and of a standard I would expect of the best zoos in Europe. Obviously when you look more closely at each collection you can find plenty of room for improvement, but they are constantly improving, and I feel the impression they make on visitors is hard to beat. Every tourist I spoke to who had visited said the zoo was among the best (if not the best) they'd visited. Night Safari and River Safari can't really be compared with anything yet, although I'll try to visit other Asian night safaris and I believe that River Safari will be an amazing park. Obviously Jurong bird park is among the world's best. I don't think I can compare it with anything I've seen before, but it's a brilliant place. I also really enjoyed the aquarium, which is certainly at least on the same level as Lisbon. If I had my own zoo and it was of WRS quality then I've be very happy. I haven't visited Sentosa's butterfly and insect kingdom, but it doesn't seem to be anything special. Underwater world may have been great years ago, but now only its unusual collection and size differentiate it from European counterparts.
So I guess it depends what you look at. The best of Singapore may be at least as good as some of the best of Europe, but there are still a few shortcomings.