Asking me to choose which I like best, beginnings or endings, is like asking me to choose my favorite child. Impossible! I love writing beginnings and endings for the same reason - they're the easiest part of the story to write!
For every book I've written, I've known how they start and how they end way before I've known anything in between. I actually write the beginning and the ending before I've written anything else - they're the bookends that have to hold the story together as I chug through all those chapters in between.
Beginnings are awesome because they can be anything in the entire world - which I love. You're not beholden to actions, timelines or anything you've set up previously (unlike chapter 17, for instance, which requires that it makes sense in the context of the 16 chapters that come before it and the ones that come after). I love that you can make up anything to start a book and it automatically makes sense because it's the point at which you're beginning. It's like the first ingredient in a recipe - you can throw anything into a pot to start, but after that things have to make sense together (you can toss in an onion to start, but don't throw in a scoop of ice cream after that! Or you can start with a scoop of ice cream, but you can't use onions as a topping, you're pretty much confined to sprinkles, fudge and the like).
Endings, on the other hand, are just as wonderful but totally different for me. If beginnings are an ecstatic giggle, endings are like a big, satisfied sigh. The characters have arrived at their destination, the issues have been addressed, all is right with the (fictional) world. One of the reasons I can't ever imagine writing a series is that an ending is an ending for me - there's no more! The meal is over, I've pushed my chair back, and it's time to say goodbye.
I'd write beginnings and endings all day, if I could. It's those pesky chapters in the middle that trip me up.