I have 2 that fall within that age group currently and both have very different personalities which in itself present unique challenges/bests. One is quiet, yet very mobile but very clingy (still transitioning) & the other from day 1 has been independant, always happy but struggles with mobility and feeding issues.
In general the bests are: The younger you get them the longer you have them (they get used to your routine)
For me the challenges with the 12-18 month age group are that they: Are not communicating yet, limited (if not walking yet) in terms of outdoor play, tend to put everything in their mouths, and a big one if you are looking at preventing long term injury to yourself is the sheer time you spend carrying (stairs)/pushing/pulling them when they are not quite mobile fully.