What I think are my top five are much different than the top 5 that I actually spend my time in:
1) DA20-100 Katana 4X by Marcel Felde
(This one would likely top my list no matter how I cut it, with easily more hours flown in it than everything else I own, across all sims, put together. I'd have to delete it and not touch it for a few years to even start to even out the totals, but it's still my regular flyer that I come back to when I'm done with the latest and greatest from whomever.)
2) R22 Beta II by Dreamfoil
(Yes, my second answer is to turn off P3D/FSX and run a different sim. Sorry.)
3) Twin Otter Extended by Aerosoft
(I don't love everything about it, and I wish that the devs would of been flexible in adding some other avionics options, but it's still one of my highest hour flyers.)
4) F/A-18E Superbug X by VRS w/ TacPack
(A pure joy to fly. I'm not even the biggest fan of the actual aircraft, compared to other, more 'historical' military jets, but it's oh-so-glorious to fly, in every respect.)
5) (blank)
(It's a complete toss up between about 10-15 other aircraft here, such as the MV B-55 & 407 & F86, the Carenado F33A & C185F, the RA Legacy 1&2, Aerosoft's F-14, Dino's F-14, Sibwings An2, Majestic'sQ400, Aerosoft's EE Lightning... Those are all pretty much a tie in terms of time spent in them, and I've more than likely forgotten a few in there.)
I largely expect that this list will start to change over the next few months. I have my eye on a few upcoming releases that, if done as well as I suspect they will be, might break into that list over time. Also, the way I've figured out my list, it's worth noting that there are many releases from the last couple of years that I own which I absolutely love, but are absent from the list because for one reason or another I haven't put much time in them yet.