Brazilian Rainbows can be great as long as you keep their temps lower (75-85 gradient) and their humidity high (80%+). They are great feeders and are the closest thing to getting a guaranteed handleable animal, although babies can be nippy the calm down fast and stay that way. They typically finish up 5-7 feet.
<img src="http://www.ssnakess.com/photopost/data/508/22tagz-sun.jpg">
Colombian Rainbows are also an option, they are smaller (4-6 feet) and less colourful than Brazilians, but have similar requirements (though can tolerate lower humidity). They are usually fairly docile as well, but occassioanlly you get a grumpy one

I don't have these in my current collection.
Hog Isle BCI are also great. They are usually fairly easy to manage, but they do have killer feeding reflexes, and while in their cage are locked in feed mode it seems

I wouldn't consider them to be unpredicatable in terms of snakes (all animals are unpredictable to some extent). They typically finish up around 4.5-6 feet, though my female below is over 6.5'.
<img src="http://www.ssnakess.com/photopost/data/508/22peanutfsoutside-med.jpg">
Colombian BCI are relatively good, as long as you are prepared for their size. They usually finish up around 6-8 feet on average. They can be variable in temperament, moreso than the above mentioned I find, but if you get a baby it should be fine.
<img src="http://www.ssnakess.com/photopost/data/508/22chasey-rock1.jpg">
Rosy Boas are some of my favourites. Cubby little buys

Very underrated. Finish up in the 2-3 foot range. Super easy to care for, slowmoving and easy to handle, come in a variety of colours and patterns. Just keep them dry! Mexican locale...
<img src="http://www.ssnakess.com/photopost/data/508/22sweetnesspumpkin1.jpg">
Sand boas can also be a good choice, though I do not have any in my collection at the time. Their care is similar to that of rosies. They finish up 1.5'-3'.
I wouldn't recommend a tree snake, such as an Amazon Tree Boa for a first snake, these are for more experienced keepers. They have exacting requirements, as well as can be quite high strung.
I wouldn't recommend BCC either, as they can be more sensitive in terms of care and temperament, and may also attain larger sizes.