Kensai is only worth getting an exotic weapon for free and then getting extra attacks with it, and at slightly worse accuracy than other similar abilities (Rapid Shot and Flurry of Blows are both -2 on all attacks to get an extra attack for example). That's partially offset by the increased accuracy though which is +1 at 1st, then +2 at 5th, +3 at 10th, +4 at 15th, and +5 at 20th. The limited feat list is also a bit of a problem for reasons that should be fairly obvious.
Pugilist is most notable for its Shake It Off ability which can be interpreted as essentially making it immune to lethal damage because it only develops nonlethal damage. Combo with some way to become immune to nonlethal damage such as the warforged feat
Improved Resiliency and it breaks the game. Besides that though, the pugilist doesn't have much going for it because its unarmed strike doesn't get a damage boost like a monk's does and thus has to rely on items, feats, and buffs (
Greater Mighty Wallop comes to mind) to boost it. The combo ability is only useful when you've got a good enough attack bonus to be reasonably sure of both getting through. It also suffers a bit from the limited fighter feat list.
Thug is nice for when you don't care about (or don't want) heavy armor, but do want more skills. It's still a pity that listen and spot aren't on the list for guard types, but oh well. The good thing about thug is it doesn't restrict your fighter feat choices so stuff like Shock Trooper can be had more easily.
If you're going fighter 20, I'd stick with thug. Kensai is better for dipping than staying in the whole way and pugilist mostly sucks at what it does unless you absolutely need to beat stuff unconscious with nonlethal damage, but even then there are better ways to do it.
Eldritch Juggernaut is bad because fighters tend to rely on buffs and SR will screw with that. That and it's way too late to be meaningful in a typical adventure. If it could be gotten somewhere in 8-12 then it might be worthwhile, but at 18? More combat asskickery can be had for the feat.
Fortification is likewise late to the party. Light fort is a +1, so if it's wanted on gear it can easily be done by 6th but if absolutely needed is doable at 4th if going by the WBL table on DMG 135. Heavy fort's +5 is another matter (and overpriced), but if absolutely needed a +6 suit of armor is 36k, so it's in the neighborhood of 11th or 12th when it might become an option if one's maximum expense on an item is 1/2 they're WBL.
