Can't think of a species I'd want to fish for every time out. A few would be off the menu around here, especially in winter rivers, if you like to catch most times out.
I like to fish different places and I'm as likely to go because I fancy a particular method, whether stick, waggler, leger, feeder,or pole, as I am for a particular species, and I like the feeling that a bite is imminent, rather than likely in the next few hours.
Since you can fish for them anywhere and on any method, I suppose I'd go for roach if pushed, especially as they reward fine gear and careful presentation. Not that other fish don't, of course.