It depends on your personal tastes really. I would try and find a shop in your area that allows you to taste or sample juices. Most shops will have tanks or drippers with juices already in them that you can taste before you buy. There are also plenty of online retailers that offer sample packs where they're small tester bottles that you can pick anywhere from 3-20 flavors to try.
Also, to change juices, you can
-fill up the tank with the new juice and let the two fuse in a weird funky mixture that you keep refilling until one taste fully takes over
-dump out the old juice and put in the new juice and the transition period is way shorter
-empty the tank and change the coil and fill the tank with your next juice to switch flavors immediately.
I would not recommend washing out the tank with water with a coil inside of it as water will get into the cotton and mess with the flavor for a while.
Some flavors like mints and menthols tend to stick in the coils and never really leave. So if you use a strong mentholated juice, everything from there on will taste slightly like menthol until you change the coil and clean the inside of the tank.
Edit: if I remember correctly, the EVOD tank is plastic inside so you might want to stay away from citrus (like orange) and cinnamon flavors as those have a history of cracking plastic tanks.
Enjoy getting off the stinkies!