Okay so I've been meaning to get back to some of these, as JB is probably my most played community server stuff. So for guards, I think that it's situational but for a general sense, here's my list
1. Sniper- Pretty obvious. His rifle can deal with most in almost any range, and some secondary choices that are nothing to scoff at. He is probably the most useful guard, ofc if he can't aim then he's as good as a bag of rocks but whatever
2. Scout- Scout get's his high tier for his speed and combat potential. He is one of the best classes for dealing with rebellers, even with ammo. "But scout sucks as mass rebels?!?" Yea, that's true. But here's the thing, mass rebels rarely happen in an lr that doesn't favor it. EG: Overheal day is pretty much asking for a mass rebel. With the exception of heavy and a few others, not many guards stand a chance. This is why this list values dealing with rebels and picks, much more common than mass rebels. Partially why engie is so high
3.Soldier- Soldier? at third? Yea, he's insane at dealing with mass rebellers and even just regular rebels, but he lacks pick potential. His rocket jumping makes up for his mobility, but still suffers a lot of health loss.
4. E n g i n e e r- Yeehaw cowboys, engineer is engihere! Why is engineer high tier you may ask? well it's simple. He's an all arounder. With a stock shotgun and pistol, no red dare step to you in the ring of honor. WIth a GS, they will be asking for the sweet release of death as your +25 hp engineer can deal with most anyone. He is much better than spy in mass rebel situations, but not as good as heavy. Anyways, he prolly my favorite jb guard.
5. Medic- OMG MEDIC?!?! well yea, medic. He's good at helping other blues and protecting the warden. Problem is, when he is dealing with mass rebels on his own, he can't do much. It's also really risky as medic is the #1 priority in regular tf2, so you draw a high target on your head. That being said, his health regen is more useful to reds but still useful nonetheless. Also uber gud
6.Spy- Pretty good at taking on picks with his revolver and knife if you can get into range, but getting into melee range of red is big bad idea. He also has bad mass rebel potential, but his combat potential is pretty good in a one v one if you can aim
7. Heavy- Poor heavy weapons guy, but yea. He's a total tank, that's about it. His health and combat potential make up for his lack of speed. His minigun ain't too bad at picking people off, but it's harder tbh.
8. Pyro- This one might be a shocker to some, but I'll be real with you. Pyro real bad at guard. His average speed but slightly higher health makes him a good base, but that's about it. Using something other than a shotgun secondary is giving up long range and pick potential. He does have pretty good combat potential though
9. Demoman-This one is pretty easy, hard to chase rebellers without sustaining heavy damage (I.E. Sticky jumping. ofc you can use the sticky jumper but then you lose combat potential). He is also incredibly hard to pick people off with, as he is a cc class.
Side note, combat potential is either dealing with mass rebels or a class with ammo or a class that is rebelling
Also this is MY list, my own opinions, and everyone can have their own opinions