Pokémon of the Week

Gengar is a viable choice on any team, mega or not. However, there are a few key differences between the two - a non-mega-evolved Gengar works more as an annoyer / utility attacker than Mega Gengar. The spooky ghost's mega form is a complete nuke, but can also be played as a Perish Song Trapper.

Focus Sash 
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name: Focus Sash
move 1: Shadow Ball
move 2: Will-O-Wisp
move 3: Taunt
move 4: Giga Drain
item: Focus Sash
evs: 4 Def / 252 SpA / 252 Spe
nature: Timid

This set is designed as a fast, powerful Pokémon that is guaranteed to attack at least once. Shadow Ball is this set's main move, hitting anything except for Dark-types super effectively. Greninja, being the most common dark-type in OU, is 2HKO'd by Giga Drain, a generation 5 tutor move. Will-O-Wisp cripples any physical attackers and Taunt cripples usual leads like Smeargle and Forretress. This set is best used as a lead.

Standard
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name: Standard
move 1: Taunt
move 2: Destiny Bond
move 3: Sludge Wave
move 4: Focus Blast
item: Gengarite
evs: 4 HP / 252 SpA / 252 Spe

As far as Mega Gengar, it's likely the best mega out there right now. This is a standard set - Sludge Wave and Focus Blast hit almost every viable choice for normal damage. Destiny Bond usually goes first unless you're picked off by priority, and Taunt cripples Stealth Rock setters and especially Chansey. Using Shadow Tag, you can switch into Chansey, Taunt, and she won't be able to escape thanks to Shadow Tag.