Assumption 1 - Nothing about me is different, I just get the item:

Sonic Screwdriver - Lots of utility there. You aren't going to take over the world with it, but it would be incredibly handy. Also have to assume you have a way to recharge it.
Phaser - Simple, straightforward, easy to use, but really only useful for stunning or killing people. Also have to assume you have a way to recharge it.
Lightsaber - Easy winner if we just go by cool factor. But seems like an easy way to cut your own leg off if you aren't careful.
Mjolnir - Right out. Requires you to either be "worthy" or to just be immortally strong (depending on whether you go by comic book Thor or mythological Thor). Regardless, I couldn't use it.
Green Lantern Ring - Probably the most powerful item in the bunch. Steep learning curve, though.
Magic Wand - Pretty much just a stick unless you are a wizard. Pass.
Pokemon Ball - No clue. The one item on the list I know nothing about.
One Ring - Invisibility, plus a laundry list of drawbacks. No thanks.

I'd go with the Green Lantern Ring.

Assumption 2 - I have whatever inherent abilities are necessary to use each item proficiently:

Sonic Screwdriver - no change
Phaser - no change
Lightsaber - a contender
Mjolnir - tempting, but not sure I want to deal with being hunted by frost giants all the time
Green Lantern Ring - no change
Magic Wand - incredible utility there. Gives the Green Lantern Ring a run for its money.
Pokemon Ball - nope, still don't know what it does
One Ring - still too much baggage.

Being a wizard vs. being Green Lantern. Hmm. Might go with the wizard just because you can wear normal clothes if you want. Green spandex and a mask just isn't my style.