Yes, the first expansion, ES, adds a lot of functionality to the game, such as Elevated Paths, Sky Tours, Jeep Tours, and Conservation Areas (although some would say those are more annoying than a feature!
). I would definitely recommend thinking about getting at least ES before MM. Especially if you think you might want ES in the future, because the expansion packs should be installed in order so if you get MM first and ES later you'd have to uninstall and reinstall everything.
Like orangemonkey stated, the Dino pack is just a Premium Download, not a whole expansion pack, so while not a big investment I wouldn't recommend it unless you do want Dinos in your zoo. I believe however it can be installed at any time so you can always decide to get it later.
AA will add Jeep Tour functionality if you didn't get ES, but other than that it pretty much just is a 20 animal expansion pack, although it does add some new behaviors even to existing animals, such as herding behaviors. Some people did feel compared to ES it wasn't as much of a bargain, but the animals included are very well made and will add a lot to your zoos if you like the animals it comes with. Again it would need to be installed before MM so if you think you might be interested in it later you might just want to get it first to possibly avoid the uninstall/reinstall process that you'd have to go through if you backtrack.
Finally, I will add that if you're a download junkie, ES and AA might be worth it to you just based on the fact that a lot of user animals are based on these models, and you would not be able to use any of the user downloads based on those XPs if you decide not to get them. (I will add that the downloads are not up right now, but will be again soon, and of course there are lots of downloads available at other sites as well.)
However if your favorite animals are marine animals, and you can only afford one XP, MM definitely might be the way to go for you! It adds the functionality of tanks, and all the MM-based user downloads would work for you. It's a great XP and adds a lot of fun to ZT2 even without the other packs.