PLEASE EXPLAIN WHY YOU CHOSE THE MOST USELESS/USEFUL SKILL IN VANILLA OBLIVION.

I choose Illusion as the most useful skill. You can kill every humanoid in a dungeon except one through frenzy. You can do the same with creatures. You can go through a dungeon just by walking with invisibility.

I choose Security as the most useless skill. If you're good at the lock picking game, then you can break open any lock, or if you have the Skeleton Key.

I chose Blade as the most useful, I chose Speechcraft as the least useful in the Main poll, and in the vanilla game.
For the question? Illusion is more useful than Merchantile overall, but in vanilla Merchantile is better than Speechcraft.
I have to add that I play a game built around FCOM, Race Balancing Project, LAME, and Integration. I actually have to invest in Speechcraft to play, but my vote reflects my opinion of the usefullness of Skills in an unmodded game.
The Security mini-game is an annoyance, and the skillset becomes pointless after a certain quest.
I chose Blade as the most useful because a Mage and a thief can always carry a dagger for backup, and the game does not have a decent selection of Blunt weapons that are light and fast.

most people would be surprised just what you can bypass with high acrobatics .
and speachcraft is the most useless for reasons you already know.