The thing which I can choose to summon, can it be smaller than my own mass or did you forget to add that condition?
This might just be me and the tiny box I think inside of but summoning and final fantasy sort of go hand in hand and I think I?ve found a summon which follows your rules, it might be a little too cute and/or effeminate but anyone who does laugh will have to use a megaphone for me to hear them from the top of my money fortress.
I?m going to go with the carbuncle, FF9 style. It can grant me a variety of abilities depending upon what sort of stone I happen to be holding. In a fight? Use a moonstone, which I assume is just a hunk of moon rock and be wrapped in a protective barrier. Need to do something discreetly? keep a diamond on your person and be made invisible, if things become too dangerous just break out your emergency emerald and run like the wind, or alternatively take the mother of all run ups and give the aggressor a kicking they won't soon forget. So in theory a seemingly innocent..whatever it is..can help me become a nigh indestructible mercenary of ultimate kickass, just as long as I only work for decent folk..Which I assume excludes the governments of the world. Chances are id have to be pretty secretive about who I am and how to contact me, kind of like a one man A-team but more successful.
The only downside is the cost associated with getting hold of these precious stones to begin with but I have a solution. I?d be able to summon a green floating..Squirrel..dog..rabbit thing, the kids love squirrel..dog..rabbit things and I?d be willing to bet their parents would shell out good money to let them go see one.
Edit: I don't actually remember any of the other games providing a summon which does anything more than reflect but if there is one which has a variety of abilities then ignore the 'ff9 style' bit of the post and assume I meant 'which generates more than one effect' I admit my memmory is failable and is composed from experience from no game before 6 or after 12.