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.
What fiesermoep said.Why wait so long to get the bump in stats?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.