Your intended use should be the final arbiter but you've unfortunately not told us if this is a CCW, home defense piece, safe queen, etc.
If it's just to fill out the collection, I'd probably go for the P5. I'm very biased towards single stack 9mms. I would be interested in the P5 primarily since it is such a beautiful piece of machinery. I personally love my P5C. The P99 is something that should be available for a while to come, but the P5 and P5C are difficult to come by, increasingly expensive, and something of a conversation starter.
If it's a budget collectable/shooter, the P1 would certainly be worth having. Considering that you can pick up a P1 for well under $450 from SOG or Century right now, it strikes me as an attractive deal.
The P99 is, from everything I've heard and read, a first rate contemporary defensive pistol. If your envisioned use is personal defense, I think I'd grudgingly give the nod to the P99. It's design, functionality with modern defensive ammunition, and durability would trump the concealability of the P5.
Just my $0.02. Please let us know what you choose.