I think one has to put some perspective into this when thinking about your fish of a lifetime.
Criteria such as; available fishing time, amount of disposable income to spend on fishing and access to venues that actually hold your 'dream' fish.
So, with that in mind, my targets would be:
1) A 15lb Barbel - this is a species that I have not targeted for about 20 years so its on my list for the next couple of seasons.
2) A 35lb Carp, preferably a Common Carp from an English water - this is one of my maintargets for this coming season, but I'd settle for a Mirror.
3) A brace of 3lb river Roach, I came so damn close at the back end of last season . . . . .
4) A brace of 15lb Trout, Brown and Rainbow.
5) A 5lb Brown Trout from the river.
So, there you are, the problem is of course that once those 'lifetime' fish have been banked then you have to raise the bar and keep going - becuase we are anglers, right?