It really depends on a lot of factors. The species of bacteria, the specific strain, the genomovar (for B. cepacia), whatever resistances the bacteria has, how much lung damage you already have, etc.
Pseudomonas is the most common "bad" bacteria. Most people have staph or MRSA; some with severe infections, some with mild to no symptoms. Those are the two most common in adult CF patients. For younger patients, stuff like H. influenzae is more common. Due to the increasing lifespan of CF patients, owing to better treatments, new bugs are becoming more common where they weren't before because CF patients would die so young. Stuff like B. cepacia, MAC/M. Abscessus, Achromobacter, etc.
One of the biggest things for how "bad" an infection is, is what antibiotics the bacteria are sensitive to. You can have a very aggressive, virulent strain of Pseudomonas, but if it's really sensitive to Tobi and Cayston, it's entirely possible to keep it in check. On the flip side, a less virulent strain of bacteria can cause chronic damage if it's resistant to a lot of antibiotics and thus harder to suppress.
Just going on average clinical outcomes, though, here's a rough idea of how "bad", from least "bad" to most "bad" the common bacteria are; these take into account general virulence as well as things like inherent antibiotic resistance and how difficult to treat or suppress they are, etc:
H. influenzae/S. pneuomoniae
MSSA (Methicillin-sensitive Staphylococcus aureus)
Pseudomonas (non-mucoid)
MRSA (Methicillin-resistant Staphylococcus aureus)
Pseudomonas (mucoid)
MAC
B. cepacia (non-genomovar III types)/M. Abscessus (probably about a tie or with M. Abscessus barely winning out as "worse")
B. cepacia (genomovar III)/some of the "new" bacterica (Achromobacter, etc.)
Extensively multi-drug resistant bacteria like Acinetobacter and Stenotrophomonas (both are "new" cf bacteria, but can often be resistant to carbapenems)
I know some people are going to be like "hey! I have B. cepacia and my friend just has MRSA and he's way sicker than me!"; these are just averages and I admit a lot of these are like comparing apples to oranges, but in general, someone that only cultures MSSA is going to be less sick than someone who cultures mucoid pseudomonas, and someone who cultures mucoid pseudomonas is generally going to be less sick than someone who cultures B. cenocepacia (B. cepacia, genomovar III).