The right show for each person strongly depends on where one lives as the best show is likely the best in your portion of the world.
That said, let me rank shows based on the largest numbers of products launched near or at them.
1. ISC West, April, Las Vegas - manufacturers tend to release more products early in the year.
2. ASIS, September, rotates in the USA - solid but lesser number of releases than ISC West
3. IFSEC, May, UK - a distant third with a handful of releases
4. Intersec, January, Dubai - very big overall show but only a fraction is surveillance plus is very Middle East / European centric
5. Essen, October, Germany (every other year) - very big overall show but very Eurocentric
Other notable shows:
Secutech, Taiwan, April - a big Asia show with tons of OEMs but few, if any, American or European manufacturers exhibiting
ISC East, NY, October - once a major show, not just a tri-state area event
There's many other shows that I've never been to in Latin America and China. I can't comment on them.
If I was a major end user or integrator and wanted to meet with the top people in the industry, I'd go to ISC West, even if I was outside the US. Otherwise, go to the biggest show in your continent.