Arg, so MANY to chose from..

Personally one of my favorites is the enhanced graphics of Skyrim, it just makes everything look better. (I prefer using the Sharpshooter mod - amazing.)

And I would choose Scenic Carriages, very nice to stroll through Skyrim in a carriage.

And just a fun one for me is Unrelenting Grip, basically you pull people towards you and hurdle them into the air.

But, there are so many different mods you can use.

I would personally check out MMOxReviews channel, he does a weekly mod series and shows the best mods of that week.
His channel: [ Register or Signin to view external links. ]

I like the naked mod ;D basically when you sneak and rob people and take their clothes they turn completely naked

Helps with lockpicking: [ Register or Signin to view external links. ]

Makes certain items weigh nothing: [ Register or Signin to view external links. ]

Never made sense that kids can't die: [ Register or Signin to view external links. ]

Get in the game faster: [ Register or Signin to view external links. ]

These are just some that I use and like. Also, if your computer can take the heat, put on some high res mods to make it really pop out.

My personal favorite is the one where you can actually kill children.

A great site to look for mods on is skyrimnexus.com you have to make an account to download larger mods, but it's totally worth it there are TONS of mods on nexus and probably is the biggest modding site for skyrim or any of the bethesda games really.