You know, I actually loved all the Halos out there in one way or another.
Halo: Combat Evolved started this whole thing of course. Without that, it wouldn't be the Halo we know. So that has some of my respect.
Halo 2 had the dual weapons. It had that going for it. Not sure why they didn't continue with that. I could see that potentially making the magnums overpowered though.
Halo 3 started the forge aspect of Halo. What isn't there to love about forge? Also you had the mammoth in the map "Sandtrap" that you can drive around with your team. Had pretty good moments with that.
Halo ODST started firefight and I really like the "big city" environment that it has in the campain. Deep down inside, I'm hoping one day to play CTF or infection in such a city as "New Mombasa" in ODST, size and all. It may take 10 minutes to get to the other side of the city, but I think it would be fun.
Halo: Reach made all things in previous Halo's before Reach better. It started the loadout system, if you can call it that. I really loved flying with the Falcons in Reach lol.
Halo 4 I had to get used too. Spartan Ops was enjoyable, although I was sad that it replaced firefight. Mantis is a unique vehicle. I was disapointed that the falcon disapered though.
Halo Wars was unique that you can control an army in a sence. It was certainly different that any other Halo.
Halo 5 I have not played yet, mainly because I do not have an xbox one, so I can't really say much. However, from what I can tell (correct me if I am misinformed), the Forge has really came a long way from where it started, let alone came a long way from Halo 4. Halo Forge now on PC. Graphics are great (partially with the help of xbox one).
If you really want me to pick a favorite, it would be between Halo: Reach or Halo 4. Halo: Reach wins for the overall game play with campain, custom games, etc. Halo 4 has a little more to offer with forge and custom games (even though Forge World in Reach was pretty darn epic). I may have chosen Halo 5 for the forge but I haven't played it yet so I wouldn't really know haha.
This is probably more than you asked for, but now you know my thoughts and my opinions about everything. You're welcome.
So recently in the last month I got myself a XB1 and Halo 5. I really enjoy every aspect of it. If I had a complaint, it would be that they could add a "Warzone" feature to customs and forge. It would be cool to go and see different "Warzone" maps that people create instead of the generic ones and makes it easier to team up with others.
Overall, I love Halo 5!