Thanks for posting the link to Legion of Super-Bloggers. I checked it out. Looks interesting and worth periodic revisiting.
As for which Legion death affected me most, I would have to go with Invisible Kid I. As I stated earlier in the thread, Superboy 203 was my first Legion comic ever. It really opened up a whole new world for me as I had no idea that this team even existed before then.
Not only was Superboy 203 MY first issue, but it was also Mike Grell's first issue as penciler (he inked Dave Cockrum in Superboy 202). Mike Grell's art introduced me to the Legion, so it will always occupy a special place for me.
In that first Mike Grell issue, it also turned out to be the death of Invisible Kid! As a kid reading this at a tender age, I was shocked that the spotlighted hero of the story ended up dead 20 pages later. I remember tearing up after reading it a second time and really processed that he wasn't coming back.
The story is even more interesting in retrospect since we now know that Validus was the son of Lightning Lad and Saturn Girl.