Upvote:3
Usually multiple-entry, long-validity visa are granted after a number of successful visits (no overstays, etc.) on shorter visas. There are always people who look at statistics which Schengen countries are most likely to grant those, but twisting your itinerary to apply at one of those countries will make the application look less coherent.
Upvote:5
Have you tried applying again? This is what happened to me, when I applied for visas (all for conferences, like yourself):
Each time, I applied for the exact duration of the conference and single entry (which is really all I needed).
In fact, I just used the same PDF application and changed the travel dates (all these visas were from the same member state).
My friend, who visits exponentially more often than I do, applied last time and got a visa for the duration of his passport validity(!) which was 5 years.