Symphony X gets a hell lot of recognition...and Redemption has a fairly large following around these parts as well.
Threshold has been around for 18 years, and I think the reason they aren't being discussed much is because it's been a while since they released an album coupled with the fact their most recent album is arguably their weakest effort.
And there is a large swathe of users here who will mock your love of Vanden Plas. I'm not one of them, but poor Norwood got killed in his Vanden Plas thread.
Bitching about the thread title aside, I rank them Redemption first, Threshold & Sy X tied, and Vanden Plas last because I've never gotten into them.
Now, if you want to talk about criminally overlooked bands that do not have a lot of recognition, here's a list for you:
Arena
Royal Hunt
Enchant
Henning Pauly / Frameshift / Chain
Tiles
and that's just off the top of my head.