I find that there are so many that you have to just use one for a while and see if you like it (kind of meta, that).
The biggest differentiators are the UI, configurability (of times, alarms, etc.), and for me, oddly, the ability to have/not have a "ticking" sound while it's going. I like it to keep me motivated, others hate it and prefer music. Sometimes I use both.
I use https://github.com/michaelvillar/timer-app and TextEdit to write my pomodoros for every day.
Take 5 minute breaks, and a big 15-30 minute break, pomodoros of size 25-30 minutes (25 mins get prolonged a little if I need to stop in a wrong moment, usually just write a note what I was doing and force myself to take a pause). I end up doing ~12 pomodoros a day.
I don't like long stretches of work, it might be just me but I end up obsessing over unnecessary details the longer I concentrate on some problems, if I take a forced pause and then continue, I seem to use less time for the same amount of work.
