there's 2 separate rankings for alakazam and kadabra
imo though it's really
alakazam > gardevoir > kadabra
what really hurts gardy is that she levels a lot slower compared to alakazam and so she won't be able to net you returns quickly, plus you need to grind from L5 to L22 for CM. abra just needs grinding for half the levels, not to mention faster level gain.
admittedly though if gardevoir was in a better exp. group, she'd be better than zam due to being fast enough (you don't really see much faster than 80), slightly better SpA and thunderbolt access.
it all comes down to whether you just want to get the game done quickly or take a slower pace: zam will come out on top since he gives you returns quickly (fully-evolved pokemon by dewford? fuck yes) while kadabra will still be able to hold its own.