My first was FL12 and then I picked up an FL20 when it became clear that my girlfriend was not going to let me use the FL12 : )
I like the FL20 a lot more for deeper water because of the bottom zoom feature, I do 90% of my fishing in 12ft bottom zoom mode and like having it oriented top to bottom also makes it quicker setting tip ups with it, etc. They both work awesome. If you're primarily fishing in less then 20ft of water, then there's not a big need for the zoom, but if you're getting into lakers and fishing deeper, it's nice to have!
The FL18 is a good compromise, I believe it has the 6' bottom zoom which is nice for perch. For crappie and lakers which can come off the bottom a little ways, the 12' is really nice to have.