So, Ranger is pretty much a 1 lvl dip(HA and FF) or an 11 lvl build. Since Gloomstalker was nerfed there really isn't any reason to go with another lvl split there, especially with Monk as Extra Attack doesn't stack.
Cleric is fine if you have a plan for what you want to do. It's going to depend somewhat on your Monk Subclass though. Light Cleric is pretty amazing as a 1 lvl dip but doesn't really synergize with Monk outside of Warding Flare. Nature can be good if you've invested heavily in Wisdom. War is probably your best bet if you want to use weapons rather than your fists and pairs pretty nicely with Shadow Monk.
That said if you are really leaning into Unarmed Open Hand Monk, you probably won't find a better multi than Spores Druid. You can stack Necrotic Damage pretty high with this combo, even in Act 2.
Not actually true, I just finished a full playthrough with Astarion as a gloomstalker/assassin and can confirm that you fet a full extra weapon attack on the first round
So, I decided to test this out since it definitely wasn't getting the extra attack at launch. My first build used Gloomstalker and I was fairly disappointed in it.
That said, as of the current patch it's actually a very good pick since instead of just the +1 attack per Attack Action on round one it's getting 3 attacks plus the Dread Ambusher special attack that also wasn't popping up on my first run. This effectively means they get to make 4 attacks on the first round of combat, 7 with Haste.
Gloomstalker Ranger 3 & 5 lvl dips are back on the menu!
The Raven Familiar is useful the whole game due to flight and it's ability to Blind enemies. The others are all situational, but still pretty good. The Cat is fantastic for Act 3 in the Lower City.