I like Hikari Ich-X. It's a formalin-based medication so it will take care of Ich, as well as some protozoans like Brooklynella and Amyloodinium, which are sometimes mistaken for ich.
Like any medication, you do NOT want to mix medicines, so that one is my first choice for the most common ailments, as well as multiple ones. Most fish seem to tolerate it well but research your fish before you buy and use any medication, as some fish don't tolerate certain meds at all.
It's a good idea to have a few different things in your "first aid kit" including Metronidazole and Focus, and perhaps an antibiotic like Kanamycin (Seachem Kanaplex). I don't recommend using the antibiotics unless you are sure you're dealing with a bacterial infection.
Fungus is pretty rare in saltwater - the salt inhibits many kinds of fungi, they *can* happen, but it's not often, in my experience.
Also good to have Prime or Alpha on hand to detoxify ammonia/nitrite, especially if using antibiotics (they don't distinguish "good" from "bad" bacteria), and a Polyfilter (made by Polybio Marine) which is handy for removing medications and toxins.
I wouldn't rely just on that to remove medications before starting a different medication (complete water change of QT water and filter media for that).
I wrote an article on QT that I left up http://www.imagine-ocean.com/best-practices-quarantine-procedures-obtaining-new-fish-specimens/">here</a>. It may answer more questions ;)
Jenn