Items

Here are the best movesets for the Pokemon you specified:

Chansey:
Seismic toss
Thunder wave
Softboiled
Aromatherapy

Ok, with chansey's moveset, you first want to paralyze your opponent using thunder wave. Next, just keep attacking with seismic toss (seismic toss will not affect a ghost, so switch if you see a ghost). If you are hurt, use softboiled to recover lost HP. If chansey or any other Pokemon on your team are inflicted with poison, paralyze, sleep, or burn, use aromatherapy to instantly heal chansey and the entire team. Remember that chansey specializes in HP and special defense. It is almost impossible to kill with special attacks. However, chansey's resistance to physical attacks is weak. Switch chansey out if you see a strong physical attacker.

Lucario:
Aura sphere
Dark pulse
Dragon pulse
Psychic

This moveset gives lucario a lot of options. You can destroy normal, rock, steel, dark types with aura sphere, hammer psychics and ghosts with dark pulse, obliterate dragons with dragon pulse, and exterminate poison and fighting types with psychic. Remember that lucario's defenses are frail, so switch out if you don't think you can attack before your opponent. Otherwise, your opponent may just hammer an attack on lucario before you can even do anything, which would be very bad because lucario is really an excellent fighter. staraptor:
Brave bird
Close combat
Return
U-turn

Staraptor's a physical pokemon. Brave bird is there for same type attack bonus, close combat eliminates dark, steel, rock, and normal types, and return can deal a lot of damage if you can't find a move that's super effective against your opponent. U-turn is listed because it's possible that you can get in a hit on a Pokemon that might be able to beat you before it attacks, and immediately switch out afterwards. Staraptor is designed to hit fast and hit hard - don't let it take any hits as it's defenses are pretty bad. rampardos:
Stone edge
Earthquake
Crunch
Rock polish

Rampardos has incredibly high attack, and bad everything else. What you want to do with this moveset is first use rock polish to boost your speed, then, just keep using the other three attacks to destroy your opponent's team. Stone edge is a great rock move that can take down flying and ice types, earthquake is a great all around physical move that hammers electric types, and crunch is for ghost and psychic types. Don't EVER let rampardos get hit. It has many weaknesses and it's defenses are unbelievably low. If you sense any danger, switch out immediately.

Ok, and now for the items:

1) Let chansey hold leftovers. It will recover it's HP steadilly, which is never a bad thing. You can find leftovers on Victory Road, after surfing across the pond in the foggy room all the way to the right. 2) Let lucario hold a life orb. The life orb powers up ALL attacks by 1.3 times it's base power. That's a lot. However, each attack you launch while holding the life orb will cost you a little HP. It's still a cool item, though. Don't get me wrong. You can find the life orb up a rock climbing wall near the entrance to Stark Mountain.

3) Let staraptor hold a choice band. The choice band increases the power of physical attacks by 1.5 times it's base power. That's a LOT. However, using the choice band means that you will have to stick with one move for the rest of the fight unless you switch out. So switch out if you don't think you can defeat your opponent. I think you can purchase the choice band at the battle tower after saving up 52 battle points. 4) Let rampardos hold the focus sash. The focus sash prevents fainting 100 percent of the time if the Pokemon holding it is at full HP. This will buy you some time to boost your speed with rock polish. You can find the focus sash at the house just to the left of pal park. There will be a man in the house who will ask you to bring him a Pokemon of a certain level. If you bring him the Pokemon he asks for, he will give you one of three items: a focus sash, a black belt, or an expert belt. If you don't receive the focus sash the first time, wait till the next day and show him the Pokemon he asks for and then he might give you the focus sash. Don't worry if it doesn't work for you the first time, this took me about three days.

Ok, I hope all that helped!

The best item for all Pokemon is the lucky egg. it doubles the exp you get in battle so if you have pokerus then your in even more luck.