I am going to say violin because I used to play. In fact as soon as I saw the title of this thread I got excited because I am actually buying a new viola within the next week myself and start playing again! If you are unsure about a violin but are leaning in that direction then a viola is your next bet, they are basically the same except a viola is just that little bit bigger and has a more mellow sound. The only problem is the range isn't as large as violins, I know what sort of viola I want but they all look pretty much the same, violins come in every colour under the sun.
Cellos are also freaking expensive, I saw some for $10,000 the other week. You can get a violin for $400.