I summon an immortal, invincible 100% loyal creature that feeds a special nurtrient into my body 24/7 through magical means. The nutrient would keep my body healthy, and give me incredibly long life, it would also be able to give these nutrients to anyone I wish it to. The creature is also extremely dangerous, and can shield me from any harm. Basically it makes me, and anyone I care for invincible.
Then there's the dick answer, I summon a creature that jams the summoning abilities of anyone in existance, this creature cannot be destroyed, and upon being summoned it removes any currently summoned creatures from our reality. It is also entirely independant from me upon being summoned, which means I cannot get rid of it, and even my death will not send it back, in short it is there for eternity (it is also immune to its own ability). It also dispenses diet pepsi... mwahahahaha!