I vote for SVN since that's what I use, but I have to admit that I really like the idea behind Git and Hg (Mercurial).
The biggest issue is usually with merging, I think, but SVN's merge tracking has made the process much easier. Plus TortoiseSVN makes managing your repo branches really simple, if you're in Windows.
For me the one feature in Git and Hg that really appeals to me is the distributedness of it all. If I'm not mistaken, your dependence on a central repository goes away, and it just makes life easier for people who are working remotely or are frequently disconnected. For example, I do all of my work in a working directory that is checked out from our SVN server. The problem is, if I don't have a network connection, I can't look at the repo or commit. This really sucks because I like to be able to check in my code at milestones, and if I'm working on an island I just can't do it, plain and simple. However, with Git and Hg the repo is local -- it's on your computer, and then if you want to later push your changes to a central repo, you can totally do that.
I still would like to move to Hg, but when deadlines are always upon you, you know what goes on the backburner.
