OK. First of all, let me enlighten you: Terran Siege Tanks DO have greater range than Protoss Reavers (in siege mode, of course). So, I think too that reavers suck a lot. The only way thay can be usefull is to put them in a (or two) shuttle and drop them in a main base (most effective against zergs).
You forgot the Parasites, my friend. They cost only 75 energy and require no additional research. Using them on the right units you can force the enemy to make some hard decisions. He can let you have his unit's sight, or he can kill one of his own valuable units.
My least favourite unit? Ultralisks (I don't want to tease you UWanaPiecAMeBoy, I still think that marines rule
). Too expensive, too large, too small damage (20dmg, equals to a dragoon attack :/ ) and they have only ground attack... I would rather build a bunch of lings and spare some gas. And we all know that zergs need a LOT of gas...