Which reaction occurs most rapidly at standard conditions?
h2(g) + i2(s) 2hi(g)
cu(s) + s(s) cus(s)
c6h12o6(s) + 6o2(g) 6co2(g) + 6h2o(g)
5c2o42-(aq) + 2mno4-(aq) + 16h+(aq) 10co2(g) + 2mn2+(aq) + 2h2o(l)
ag+(aq) + i-(aq) agi2(s)

 I don't know of any way to determine the relative rates of reactions other than pretty much just guessing. Even with guessing, I can't decide between the last two. Both occur in solution and (if I recall correctly) both occur quite rapidly. I would guess that the last one, just a simple precipitation, would probably be the fastest. 

(NOTE: You have the wrong equation on the last one. It should be Ag+(aq) + I-(aq) --> AgI(s) There is no subscript on the iodide ion in AgI.)