Gengar has a HUGE support movepool. Both Gengar and Alakazam are outclassed as all out attackers by Greninja. However, Gengar can run plenty of support moves such as Will o Wisp, Destiny Bond, Taunt, Perish Song, Haze, Trick, Disable, etc; many of which are uncommon. And he can run the support moves alongside Shadow Ball and your choice of coverage moves.
Oh, and for those of you who miss Mega Gengar, this set is a bit gimmicky but does a ton of work:
Gengar @ Black Sludge
Ability: Levitate
EVs: 32 HP / 144 SAtk / 80 SDef / 252 Spd
Timid Nature
IVs: 0 Atk
- Substitute
- Perish Song
- Infestation
- Shadow Ball
EV's are to maximize speed and prevent defensive Mega Venusaur from breaking his subs. This set naturally lures Chansey, under the threat of wrecking something with a Shadow Ball. If the opponent gives you one free turn with whatever they switch into you, such as using a healing move or setting rocks, it dies. It also can be used to eliminate passive setup sweepers like Suicune and Clefable, since many will try to set up on you under the threat of Shadow Ball or a coverage option.
http://replay.pokemonshowdown.com/ou-128883268 <-- skip to turn 13
http://replay.pokemonshowdown.com/ou-128998637 <-- skip to turn 20
Perish Song is also one of the best ways to put a stop to Baton Pass/Quick Pass teams, even more so if they don't see it coming.