Go Pact of the Chain and take a Pixie. Honestly... if you want a familiar that adds a ridiculous amount of power, get a pixie. It's ridiculously overpowerd. It can spam greater invisibility, so its one flaw (1 hp) isn't that big of a flaw, because if the enemy knows where you rpixie is to kill it, you've messed up. With a pixie you get a druidcraft cantrip and 1/day: confusion, dancing lights, detect good/evil; detect thoughts; dispel magic; entangle, fly, phantasmal force, polymorph, sleep. So you can attack while your pixie drops sleep on a bunch of mooks, or polymorph you into a CR Level equivalent beast ... like a tyranosaurus rex for example. Or fancy flying? The DC is only 12 for the save, but just the three mentioned make it worth it alone.