Castillo was very tough. JuanMa's skills mean it is tough for him to be counted out against anybody, but JLC was one tough bastard with good skills and he was a big LW. I think prime on prime Castillo might be able to wear him down. Not definately though.
Diaz cut off the ring and had Marquez trapped on the ropes, Jose did the same thing to Floyd, if jose did that to Juan, Marquez would be done for, thats a lot of weight in those punches
Before factoring in size I would say JMM. JMM would be able to keep the distance and box Castillo well, countering many of his shots. I don't think he could hurt a prime Castillo but if he fought a smart fight should be able to outbox him for 12 rounds as long as he doesnt get into a slugfest with him. All that being said Castillo may have been too big for him at the end of the day. But based on skills JMM for sure.
Castillo would uppercut his head off, the lightweights now aint like the lightweights back in 03-05, Marquez's punches would feel like snow hitting Castillo's ugly a$$ chin