As with anything a lot depends on how interested you are & Best is subjective.
For Windows?, things I have used in the past.
DVDslideshow gui, this is a front end for a lot of stuff, so it will download all sorts, avisynth, ffmpeg etc. All valid stuff nothing to be worried about. It has more transitions than you will ever need. It says it can now incorporate video clips. I have only used it for images. http://download.videohelp.com/tin2tin/
Photofilmstrip. A nice simple application, that can zoom in (ken burns effect) on an image.http://www.photofilmstrip.org/2-1-Features.html
The page starts in German, language toggle is very small, top right corner.
Foto2Avi, I did try this one relatively recently, and can't remember much about it other than it works.http://www.videohelp.com/tools/Foto2Avi
Thats the videohelp site and is worth a trawl around for all things video.
start here: http://www.videohelp.com/tools
Avidemux, Simple NLE if you need to chop-n-join or change the audio of your slideshows. Can do simple effects such as adding subtitles, logos.http://avidemux.razorbyte.com.au/