Ah yes, Alakazam... the original special sweeper. A nice Pokemon overall, with great stats in place for sweeping. Unfortunately, as we all know, it's defenses are a bit lackluster, so it's new ability is almost worthless since most will go for direct attacks against it.
The Magician
Modest @ Choice Scarf
Inner Focus/ Synchronize
-Trick
-Guard Split
-Psychic
-Signal Beam
252Sa, 252Spe, 4Hp
This set is designed to work as a lead for Alakazam. Usually people will send out thier defensive one in single battle first, such as Forretress and the unbelievebly common Ferrothorn. With this set, you can Trick them and make those slow bulky types hold an item that lets them use only one attack. From there, go with Guard Split, which switches thier defense. Guard Split turns Alakazam's horrid defense into an advantage- it will gain defense and at the same time the opponent will probably lose some. From there, proceed to sweep with that 405 special attack.
The Original Special Sweeper
Timid @ Life Orb
Inner Focus
-Psyshock
-Shadow Ball
-Focus Blast/Signal Beam
-Energy Ball/Shock Wave
252Sa, 252Spe, 4Hp
The original special sweeper with a few new twists. Firstly it has the life orb, which will boost all its attacks 130%. Secondly, it now had Psyshock, which is perfect for dealing with the ones who are specially defensive (Blissey and Chansey hint hint). Everything else is for coverage. Focus Blast or Signal beam, depending on which type of dark destroying attack you prefer. Shock Wave or Energy Ball for water types
Other moves: Thunder Wave, Magic Coat, Encore
Thunder Wave has many applications, Magic coat can be good if you time it coorectly, and Encore is just plain awesome for set ups. On a side note, you can also use the item Focus Sash to make up for its poor defenses.
Counters: Anything with a priority move, especially Shadow Sneak or Extremespeed. Spiritomb because it can have Shadow Sneak and high defenses, while maintaining no weaknesses. Arcanine would also work given Extremespeed and its high attack stat. Anything faster than it can also defeat it in one shot- Gengar for example can Shadow Ball it easily.