Unless they specifically like one with hearts don't get one of those. Most girls I know really dislike heart necklaces and only wear them because they feel obliged to after their boyfriends bought them one.
Well I like them all, but if I had to pick just one, I'd go for number 4, then 2. Both elegant and unusual, 4 is more classic I think though, 2 may not age very well fashion-wise.
1 is nice, but you see it everywhere and the design is always ripped off. 3 is nice (and I wouldn't say no), but could be a little large and possibly not to everyone's taste. 5 is the same as 1, although I think it would look much nicer on a finer chain.
I have a necklace similar to 1, definitely not a Tiffany's though! I think that kind only look really nice on the right thickness/length of chain which depends on personal preference. (The chain on mine is too short so it looks a bit silly.)