This is going to be extremely biased but I absolutely loved the first Halo, it will remain one of my all time favorite shooters as it was the one that got me into playing them (yea I'm a huge Halo fanboy, it was the main reason I got an XBox). Yea the story is rather cliche in most parts but the original Halo was still a great game that had a rather engaging story (if you want more on my opinion go read the old review I did for it in the archives).
The second Halo is by far one of their greatest flops of all time for me. Yea it did wonders for multiplayer but as a single player game it sucked. They tried to do too much in such a small amount of time that it was honestly a cluster fuck of a game. They introduced a second character (the Arbiter) who's story basically killed the game for me as his story was bland and uninteresting compared to what John-117 was doing. The weapons were severely nerfed from what they were in Halo and gah I just don't know how else to explain it.
Halo 3 (yea I know you asked for the first two but lets run down the line to give them a little more clarity on their game making) was a good game, not great and not terrible but good. It did a good job of wrapping up the story as a whole and did much to redeem Bungie for me after the horrible game that was Halo 2 but it wasn't as good as the first one for me. It was still a fun game.
Halo 3: ODST was a great game for what it was. My love for this game is mostly cemented for the fact that they used Nathon Fillion, Alan Tudyk, and Adam Baldwin as voice actors and well we all know I love Firefly so yeah. The game had a good story, great characters, and was extremely fun to play (and a bitch to get through on Legendary).
Reach is another game that I loved. A lot of people hated it but I on the other hand loved every minute of it (I say that without having beaten it on Legendary). It gave me back all the weapons I loved, introduced me to my favorite Halo weapon (the DMR), let me see how Reach fell (one of my major interests as a Halo Lore buff), and introduced us to Spartans that actually had more emotions then John-117. Although I already knew how the game was going to end the story was still extremely engaging. Honestly, after Halo, Reach is my second favorite game in the series.
So yea Bungie has a history of ups and downs and it seems they're on a downward spiral right now with Destiny. They're trying to do so much with this game and sadly are getting nowhere with it because they don't know how to deliver everything they had promised while still balancing everything for both sets of players (Hardcore and Casual). Sadly they've been leaning more towards keeping their Hardcore players and giving us casual players the shaft around every corner. They need to go back and look at what they did with the Halo games to learn where they're going wrong with Destiny as well as, as much as I hate to say this as they're not the greatest examples of development team awesomeness, get with Blizzard and figure out how to make this game deliver on the casual content.