I don't want to heavily invest in class decks as my friend has RotR and has bought quite a few already. I like the magical/mystical theme around the base characters and add-on characters but I wanted to try out some unique class decks like Warpriest and Gunslinger.
Any suggestions on fun, worthwhile class decks?
Do you want to mix the class decks into the box or play the characters (or both?)
Gunslinger will probably have a hard time, since they really want those guns, and they'll most likely be rare if you just mix in their deck without removing other weapons from the box.
In general, the more recent a class deck is, the better and more interesting unique boons it has. Summoner and warpriest both seem like cool choices, and monk brings many exotic boons while making sense in the setting.
Do you want to mix the class decks into the box or play the characters (or both?)
Gunslinger will probably have a hard time, since they really want those guns, and they'll most likely be rare if you just mix in their deck without removing other weapons from the box.
In general, the more recent a class deck is, the better and more interesting unique boons it has. Summoner and warpriest both seem like cool choices, and monk brings many exotic boons while making sense in the setting.
I guess it would depend on which class. The weapon rarity with the Gunslinger makes sense, now that you've said that. If the Summoner, Warpriest and Monk classes mix into the game well, I don't have a problem with that.
Your best bet would be to look at the card list on the product page of each class deck to see what sounds cool to you. The first seven class decks (Fighter, Rogue, Wizard, Cleric, Bard, Ranger, Sorcerer) borrow heavily from Rise of the Runelords, so they might not be very exciting, but everything after that has their fair share of new stuff.
Since you are playing MM, you could also make the following houserule to make characters work that depend overly on the cards in their class deck:
Don't mix in the class deck cards; instead, make a class deck trader availabe that exclusively draws cards from the class deck of the scenarios adventure deck number or lower, and set the price to 2-3 boons of the same type. You may tweak that houserule as you see fit in terms of costs and number of boons offered.
If you want to play a gunslinger and feel like mixing in guns makes them too rare, you can make it work with some houserules like this, so you should really go for flavor.
In general, its probably better to just go after the class decks that sound fun to you, judging from their concept and the card list. Every class deck after the first seven usually has two unique themes of cards that are exclusive to the card decks. You can also look at the charactersheets in the paizo community ressources for free, to see what characters look interesting. The most recent ones also have some roles that can interact with MM (Uliah from the Warpriest deck can interact with the trigger trait for example)