Favourite In-battle effect held item?

[Tornado]

In-battle effect item - Bulbapedia, the community-driven Pokémon encyclopedia

My favourite is Rocky Helmet.

 

[ToppoNeko]

I like the Air Balloon!
Heatran on a balloon, FTW!

 

[Codface]

The king of held items has to be leftovers. Whether for saving pots through a long cave, tanking a legendary while spamming pokeballs or being the best item for any number of defensive walls.

 

[UnovaCastaway]

The king of held items has to be leftovers. Whether for saving pots through a long cave, tanking a legendary while spamming pokeballs or being the best item for any number of defensive walls.

I LOVE Leftovers too, for similar reasons.

Rocky Helmet is my second favorite, and I usually have an Amulet Coin on one of my party members ingame.

 

[Voltaire Magneton]

I like Exp. Share and Amulet Coin because how convenient they are.

 

[Berrenta]

My faves are Leftovers, Shell Bell, Rocky Helmet, and Exp. Share.

I also use Power items on occasion when EV training.

 

[smalllady]

Lucky Egg. Less time is needed to level up with it, especially in Gen V.

 

[Yohanesu]

Left-overs .... XD

 

[RainbowSylveon]

I would normally say Leftovers, but after leaving a Macho Brace on my Druddigon for most of White 2 (I honestly forgot it was there after a while xD), that's my new favorite. Druddigon's Attack stat was fantastic once I took it off.

 

[Almonds]

White Herb. Really useful after my Gallade uses Close Combat.

 

[Dragonfyre]

Leftovers, AKA the king of saving your sorry hide. I'm fairly sure that if you put it to a vote, it would win. Everybody uses Leftovers.

After Leftovers, my favorites would have to be the Choice items.

 

[Lofty]

The Exp. Share and Lucky Egg are SO. DANG. USEFUL. Especially when you've just caught a new 'mon that's WAY underleveled for your team.

 

[Silverwynde]

Leftovers: for the HP restoring.
Lucky Egg: to help level up some of the lagging members of my team.
Amulet Coin: to help boost the finances.
Macho Brace: to help EV train some of my take-ins and bring out their inner strength.

 

[Jyo]

Lucky Egg and Exp. Share is my perfect combination. It makes the leader of the team much stronger, and the exp. share I'll let the Pokemon trailing behind the most get a chance to catch up without battling.

I also like the Amulet Coin. Rocky Helmet is also another held item I like to use a lot.

 

[Fudgenuggets]

Y'all be trippin' balls. Iron balls.

 

[Kojo]

I like Rocky Helmet too. I laugh whenever I use it on Ferrothorn and the opponent loses 1/3rd of their HP just by using a contact move.

 

[Mami Tomoe]

Life Orb hands own. Trading your own life to deal more damage? That's just godly, not to mention kinda epic and/or heroic. :3
edit: Leftovers deserve a honorable mention too, for obvious reasons. That extra 6% hp per turn is just asdf. <3