most effective bass traps

Some get there traps tested at independent third party laboratories - such as GIK. This provides conclusive evidence of their effectiveness - how they work in your specific room is of course highly dependent on your specific issues and placement.

Large thick broadband corner absorbers are the most sensible approach to bass trapping, IMHO. (Fibreglass or rock wool works well but appropriate density foam can be good too....size does matter in acoustic absorption)

As an aside, I enjoy your well thought out responses to many questions on this forum. Am I correct to assume that using corner traps, such as ASC tube traps, is virtually always likely to improve bass response?

Bflowers,

Yes, for sure! Sorry I missed your question. It is extremely rare to have too much bass absorption. These are broadband absorbers and NOT "tuned tube" traps (tuned helmholtz resonators to a specific frequency - like your beer bottle). They appear similar to other broadband absorbers in the way they function - so you can place many of them in a room without fear.