Whopa Gengar style

Still working for a SC/SB one. I have a 91.1% one and its nice!.

Only decent move for Gengar is SC/Sludgebomb
Try again until you get it.

But even with this move, Gengar is worse than Alakazam for everything.

1 or 3. Although personally I wouldn't invest into a Gengar in the first place, I'd just use #1 as it is currently. If I wanted a glass-cannon attacker I'd work on an Alakazam instead. Gengar doesn't have any niche use as an SE attacker because the types it is strong against also deal extra damage to Gengar, so I'd rather have Alakazam with Psycho Cut which is only second to a DB/DC Dragonite for neutral DPS.

Sucker Punch is actually a decent move. It doesn't have STAB but the lower cooldown means you're likely to get in extra attacks in between dodges which make up for the damage that Shadow Claw has over Sucker Punch.

You lose more DPS from Shadow Claw than Sucker Punch in actual combat because you need to dodge as Gengar, so you cannot spam attacks off every cooldown, which means you end up losing a lot of DPS with Shadow Claw.

For example if you're against Water Gun or a similar 0.5s cd move you can fit in 2 Sucker Punches (14) while you can only get in 1 Shadow Claw (13) and you waste about half a second because it's not fast enough to get a second attack in.

For ~1 second moves, you can fit in 3 Sucker Punches (21) or 2 Shadow Claws (26) before dodging. The damage is similar in reality because of rounding and because each move gets +1 damage added, so quicker moves are better on offence.

Personally I'd rate Sucker Punch + Sludge Bomb over Shadow Claw + Sludge Bomb, because both moves deal comparable damage when you consider the fact that you need to dodge, but Sucker Punch is less clunky to use.