There are many weird and wonderful words whose use I enjoy from time to time. However, for a number of years I have extracted a great deal of enjoyment from the use of the words 'absquatulation' and 'discombobulate'. I also like 'counterveil' and 'panjandrum'.
I often insert them into sentences and enjoy the looks on peoples' faces.
Pioneer is one that comes to mind. Least favorite word is literally as everyone seems to be misusing/abusing/overusing the word all the time.