Simply putting it, no. In terms of base stat, several Pokemon have higher base stat such Darmanitan-Zen, Delphox, Gallade, Gardevoir, Espeon, Exegutor and Starmie. It is the fastest non-legendary Psychic type, but apart from that all it's other stats is lower than something else.
Alakazam does have a great movepool, but it have trouble utilising it. It's physical movepool is in essence useless due to its low Atk stat, and it is sorely lacking key setup moves like Nasty Plot. This lack of worthwhile moves makes Alakazam very predictable, despite good coverage.
A moveset would often look similar to this:
Alakazam @ Focus Sash/Life Orb
Ability: Magic Guard
EVs: 4 HP / 252 SpA / 252 Spe
- Dazzling Gleam
- Shadow Ball
- Focus Blast/ Hidden Power Ice/ Grass Knot
It can of course use the Choice Items, but they makes him either to slow or not powerful enough, making LO/FS more advisable.
It faces stiff competition from the likes of Espeon, Gardevoir, Gothitelle, Latios, Latias, Mew, Slowbro and Medicham in terms of pure utility and as the best Psychic type. All these Pokemon have something that makes them special, whilst Alakazam is not much more than a ok revenge killer. Granted, it is still a good revenge killer, something that Slowbro could never be for example, but Slowbro is still better. Since it is a superb wall and Alakazam is merely a good sweeper, Slowbro would be the overall better Pokemon, despite fullfill ign very different roles. Another example of this is Mew. It's near unique capacity to counter Medicham makes it almost one of a kind, whilst there is dozens of good sweepers of other types that could do Alakazam's job.
The abundance of Dark and Bug types have severely hindered Alakazam, and with everything running Knock Off Alakazam is having a hard time even getting into the match. The likes of Bisharp, Crawdunt, Scizor and Tyranitar with moves such as Sucker Punch and Pursuit simply makes Alakazam vulnerable.
Alakazam does have a Mega, but not even that makes it outstanding. Albeit it possesses high Speed and SpAtk, it is still very frail and sorely miss the extra survivability or power provided by Focus Sash or Life Orb. As it is so frail, it often can't sweep as it gets outspeeded by Choice Scarf and can't take down truly bulky foes.
Alakazam is certainly a good Pokemon, but it isn't the best.
So no, Alakazam isn't the most powerful Psychic type, but there really is no most powerful Psychic type, or Pokemon overall. All have different roles and perks, making each more or less suited for the needs of each trainer or team.