My favorite tank was the Exterminator, but I don't know anymore. I like my Demolisher. I also like my Manticore.
As for having a lack of names for the Shadow/Storm Sword, I find those names somewhat appropriate and not for lack of a better name. Think about it. The ShadowSword (Beyond being painted a shadow grey in the pictures) can hit most things likely before they can even see it, striking from the shadows, slicing through the opponents armour like a large sword.
The StormSword is a refit of the ShadowSword, and mounts a gun that has the equivalent potential of a Medusa Siege Mortar. It can 'Storm' a fortified position. (Note: I don't have any official information about the StormSword's main gun, but it looks like a Long Barreled Demolisher, just like the Medusa has.)
And one last thing. Delone, the question on this specific topic is "What do you think is the best tank?" not "...best Imperial tank?" or "...best Eldar tank?" Just "best tank." So, if someone thinks the Wave Serpent is the best tank (of which arguably, it very well could be), they have the right to post that opinion here. Just like someone posted "Land Raider Crusader" there somewhere (No, it's not cheesy, it's 255 points). If this forum didn't want an eldar presence, they wouldn't have asked ELDARonline to host it.