Alakazam has been one of the greatest sweepers since Generation I when psychic pokemon were superior to all other types in the game. times have changes and modifications have been made so that psychic types no longer have an unfair advantage. However, Alakazam with the right moveset can still be an unbelievable powerhouse in the OU tier.
when using Alakazam, you should know hat he is incredibly frail and can be OHKOed by most moves in the tier. however, Alakazam makes up for this by being one of the fastest and most powerful pokemon. Alakazam can almost always hit first and has a movepool that is full of variety, allowing him to have great coverage and OHKO almost any pokemon before they can even touch him.
the best movest to give an Alakazam is one STAB psychic typed move suck as Psychic/Psyshock for an incredible hit. if it is super-effective, there is no way the opponent can survive. if it isnt, it will most likely, only take 2 hits.
then to battle other psychic types such as himself, equip Alakazam with Shadow Ball/Dark Pulse to ensure that your opponent doesnt try and use your own strategy against you.
Another must for Alakazam is Hidden Power. Hidden Power can be any type of your choice and you should choose the type that is super-effective against any type that seems to show up frequently such as steel and dark.
Finally, in order to KO pokemon like Heatran, and Tyranitar you'll have to use a risky move. Focus Blast is a bit unnaccurate but if it hits, you may just win the battle.
Alakazam can be used in just about any team since with his Magic Guard ability, he only takes damage from attacks. no need to worry about Hax with Alakazam or damage from weather or hazards.
The items that are perfect for Alakazam are:
Focus sash, since he might not KO his opponent he will stay alive and finish the job next turn.
Life Orb, boosts his power without the suffering that other pokemon have to go through.
Choice Specs, one of the most poular; makes him incredibly powerful and enhances the chance of OHKOing his opponents with just about any move.