There are at least 4 items that can add 1 to your crit chance + the Champion ability. What I found:
Dark justicar helmet (must be obscured) (Helmet)
Covert cowl (must be obscurred) (Helmet)
Shade slayer cloak (must be hidden)
(Unconditional)
Lvl 3 Fighter (champion)
Saravoks helm. (Helmet)
Blade of the first blood
The dead shot
They all reduce the number necessary to get a critical with no caveats. They stack regardless of status, position, or concealment.
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Knife of the Undermountain King
Duelists rapier (must be the only melee weapon equipped)
Both if these make a crit on a 19 or 20 roll. These stack with the above options but do not stack with one another.
So all together, I think, you could couple the following:
Lvl 3 Fighter (champion)
Saravoks helm. (Helmet)
Blade of the first blood
The dead shot
Knife of the Undermountain King
...for a total of +5 to crit or crit'ing on a 15-20.
