Welcome to PokéCommunity! Register now and join one of the best fan communities on the 'net to talk Pokémon and more! We are not affiliated with The Pokémon Company or Nintendo.
I would personally choose leech seed, since you really need sunny day in effect to get the best out of synthesis otherwise it is a waste of an attack.
If your team is constantly swapping out then go for intimidate as that extra lowering of attack could save you the game as with leech seed.
At...
I would not teach the elemental punches to an Alakazam anymore due to them being physical moves and well ... Alakazam sucks in the attack stat.
Use special moves for gengar and alakazam and to add veriation use your Infernape as a physical sweeper with move types that your gengar and alakazam...