Having played all bosses here is my rankings:

TOP TIER:

1.
Goldie - special skill is almost game breaking. Very strong empire skills too.

2.
Sai, insane special skill, bane of bosses, especially after getting the " debuff last longer" skill. Better yet, you can clear thug houses in 1 round of combat with it. Poison is STRONG. Chuck em near doors and force everybody to walk through the cloud. After this they can move max 2 spaces, and barely take actions. Average empire skills.


STILL REALLY STRONG:

3.
Mabel Ryley, quite good special skill upper tier, just not sai or goldie. Excellent empire skills & synergy.

4.
Daniel Jackson, strong boss skill, just needs a bit of setup (range). Excellent empire bonus, that 20% casino money is pretty big.

5.
O'Bannion, decent boss skill, his empire skills are pretty good. Also the -war negativity is a bonus, to a good boss already.


AVERAGE:

6.
Donovan, I dont like melee skills. Good empire skills, better Frank, hence me placing him above Frank.

7.
Frank, I dont like melee skills. Good empire skills carry him.

8.
Genna, OKish boss skill, need to get that debuff skill to make it better. Also starts with a shoot first skill, which helps! OKish empire skills

9.
Saltis, weak boss skill, Excellent empire skills, making it easy to ramp up fast.

10.
Capone, OKish boss skill, good for clearing weak enemies fast. decent empire skills.


TRASH TIER:

11.
St Claire - almost useless skill, OKish empire skills

12.
Elvira - fun skill, but nothing game changing, really weak empire skills.


WORST TIER:

13.
Salazar, weaker pistol skill than daniel, nice casino skill, but very very weak otherwise.

14.
Dyer - trash skill, meh empire skill. Just avoid.