IMO the best style of airer is a table airer/balcony airer - they are ubiquitous in Italy and Germany (to name the two places I have seen them...) where living in an apartment is normal.
I like them because: 1. you can peg your washing to them, so it doesn't blow away when you put it outside 2. you can move it indoors/outdoors easily if it starts to rain/the sun comes out 3. the washing cannot overlap the other washing as there is only one 'layer' of hanging space 4. I can fold in the wings and put folded, line-dried washing on top to finish off in front of the woodburner in the winter 5. delicate items can be pegged over one or more rungs to facilitate speedier drying with no damage 6. or placed flat like the above-mentioned line-dried washing
You do need to make sure you buy a big enough one though, as the actual hanging soace can vary considerably - ikea do one, ours is from betterware, although the 'wings' have bent a little under the weight of the washing so i wouldn't recommend ours, and you can get increasingly expensive ones from Leifheit.
I'm sure you can tell i've spent a lot of time thinking about this!!!