As others have said, probably window units will be the most effective.
From
what I've seen, a single window unit works pretty well on smaller
units, like studio or 1-bedroom apartments. Once the units get bigger,
you may need something like one window unit per bedroom; people don't
always want to leave all the inside doors open so one unit can cool the whole
place.
If there is more than one window unit per unit, and one of
them craps out, you usually have the option of shuffling a working one
into the bedroom(s) and then getting a new window unit tomorrow. In the
summer, Wal-Mart (24 hours a day) usually has some in stock; I'm not
sure if they stock them all year, though. (This assumes you provide the window units.)
You do have to clean the
filters on them. Often this isn't a paper throwaway filter like a
central A/C has, but a section of wire or plastic mesh in the front
grille that you can pull out and wash. You might consider buying spare
filters, so when you go for a maintenance visit, you can just swap in a
clean one and take the old one back with you to wash later.
They
should tilt towards the outside of the house a little bit, so the
condensation drips out the back. Normally this happens "automatically"
when they are mounted in the window, but it's a good thing to check
every couple of years, so they don't start dripping inside the house.
If
the unit ends up dripping somewhere you don't want it to, some units
have a place you can connect a drain pipe/hose and move the drip over a
few feet. You don't usually plumb this into a drain; you just use a few
feet of pipe or hose to get the drip away from the doorway or whatever.
If
the unit has more than one bedroom, it's kind of common for the outlets
in two of the bedrooms to be on the same circuit. Two window units on
one circuit will usually be OK, but it doesn't leave a lot of room for
the tenants' stuff. You can usually run a few lamps, alarm clocks,
phone chargers, etc no problem, but if the tenant wants to (say) use a
hair dryer in the bedroom, they might end up popping a breaker.
If you have to run a new circuit just for the window unit(s), consider putting a single outlet next to each unit, instead of the common duplex outlet. If you think you need a really big window unit for some reason, running a new circuit usually lets you use a 240 V window unit if you want.
One
disadvantage to a window unit, especially if it's on the first floor in
a less-than-great neighborhood, is that it's relatively easy for the
bad guys to push it into the house from the outside, and then climb in
through the resulting hole. Most bad guys aren't bold enough to try
this when someone is home, but I've heard of it happening when the
tenants were away, or when the unit was vacant between tenants.
If you have a bedroom with only one window, and that window serves as an egress window (second exit in case of fire), check with the local building-code people to see if you're allowed to put a window unit in that bedroom. Sometimes you can and sometimes you can't.