Answer:
For me its all railways because if you have them all, then you get price of one railway when someone goes on it, blue streets because with hotels they gives 2000 and 1500, which is very good, then goes brown (or purple in other versions) cause its only 500 to make hotels on both of them, and last Yellow! yellow is good because it is visited very often and nobody buys it some why (really! i played monopoly like 500 times, lost only 2 times in my life and nobody bought yellows).