Gen I and Gen II -Alakazam....It was a complete special juggernaut back in the good 'ol days. It got crits often and it was a sweeper as well as a tank. The dream pokemon.
Gen III- It got some competion in Xatu, Claydol and Hypno but it was still pretty good. The split between special attack and defense didnt treat it very well.
Gen IV- Now there are special and physical moves? Oh crap. Now alakazam is going downhill. A little more competion but not a whole lot. And an increased movepool helped it some.
Gen V- The introduction of super powerful darks and more powerful psychics left alakazam in the dust with is speed now average. This classic pokemon's glory days are over in the eyes of most trainers.