I'll go with the five 3rd party products option :)
1/ (The Slumbering Tsar Saga ) by Frog God Games. As well as being an amazing work of evil genius, it was my gateway into 3PP and specifically Frog God Games. I owe every purchase below to this book leading me into a world much more like the way I want to play games.
2/ Swords and Wizardry Complete. A retro clone of the original version of D&D plus the first few expansions. I bought the hardback but you can get the PDF free from Frog God Games. I love it so much! I'm 5 games into running a campaign, there have been 5 PC deaths and the players are loving it.
3/ Cyclopean Deeps. by Matt Finch of Frog God Games It's a bizarre subterranean sandbox. It's not Drow or the other typical stuff - it's where the Drow fear to tread and it is awesome. Pathfinder or Swords and Wizardry. The only problem is that it's for level 12s, which is high level in Swords and WIzardry so I don't know if I'll have a campaign make it that far. If you're playing Pathfinder then I'd strongly recommend checking it out. The world is really interesting.
4/ Shadowbrook Manor by Goblinoid Games . Such a fun, creepy mansion module. I'm running two groups through it currently and it has given some beautiful moments. Such as 3 of the party of 7 being temporarily magically psychotic, the remaining 4 players having to drag the three tied up PCs along the marble floors to the Mansions doors, opening the doors and being greeted by bow-shot from the darkness. The fear in the players was fantastic but it's not a mean module - if the players are sensible (and not unlucky) they'll survive. To my shock nobody actually died in that nightmare situation I described above, but they certainly had a tense half an hour (real time) hiding and waiting for their comrades to snap out of it.
5/ The Lost City of Barakas by Frog God Games. A sandbox campaign setting for levels 2 to 6 (or higher). Pathfinder or Swords and Wizardry. There's a good town setting, and a nice deep and nasty dungeon, plus surrounding wilderness encounters. This is where I intend to continue my current S&W campaign - the players have of course got side-tracked by various other stuff first.
My major publisher purchase of the year is D&D 5e. The three core books. I think I've burnt out a bit on Pathfinder's crunchiness after playing through the entire Rise of the Runelords AP this year (which was very fun, to be clear!). 5e is a nice middle ground which I'm really enjoying.
I know the above is a ridiculously Frog God Games-centric. They really make the kind of material that I love and they were my intro to 3PP via Slumbering Tsar. That said, I've got some Lamentation of the Flame Princess modules en-route, the Carcosa campaign setting (dark! maybe I'll only read it, but if I really like it then maybe my S&W campaign may find a portal to go to a place even more dark and dangerous than the Lost Lands). I've also bought quite a few modules from other companies too, many of which were good, but apart from Shadowbrook Manor none of them make my top 5 for the year.
And one final great (free) product for 2015! The webpage that I made to make it much easier for Pathfinder monster summoners to get creature statistics, with or without augment summon or the summoning templates - http://gregfarrell.org/pfs/ . Monitoring the thread I posted about that earlier this year in this forum is what led me to learning about Slumbering Tsar and the wonderful world of 3PP :)
I'll go check out Legendary Games now too :)