Well so far I think Ive had pretty much every type of fish suitable for small to medium tanks..
Blennys are grest fun, I have a scooter blenny and a bi colour blenny. The bicolours are fab fish with lots of charachter. they are good eaters and also very peacfull fish. Add both colour and movment to a tank. The Scooter again is a fab fish but you have to make sure you get one that eats frozen food. Again very peacfull and interesting fish.
Gobbies.. most gobies are suitable for small to medium tanks and are peacfull. But if you want a goby that is more seen around the tank you want a clown goby. These come in a few colours from green to red and yellow and also black. They can be picky eaters and you have to make sure that they get some nosh at food time and dont get out competed.
Clowns are great but again these can sometimes be a challenge to get to eat and settle. Live food I find gets over this problem very quickly.
Jawfish are a peacfull and interesting fish to have, but you need a good deep sand bed and a mixture of sand and rock rubble for them to build a burrow. Again these are good eaters and will take most frozen fooods that are offered.
Avoid all damsels.. not good in a small tank as they get nasty with age.!!
You can get some nice wrasse fish now that will go well in a small tank. I would avoid the six line though as they are a bit grumpy in a small tank. They tend to be good eaters and add colour and interst to a tank.
Cardinals again are a good medium sized tank fish and very peacful.
All of the above fish are reef safe and usually quite hardy. But it goes without saying always ask about the fish before you buy.. Whats it eating , how long you had it in, whats the tank condition like I.e. PH etc as that will influence how long you aclimatise for etc. And always buy the healtiest of fish, dont just buy it cos you feel sorry for it or it looks pretty. Make sure it will fit in with your others.