The Masters of the Wild string of Feats were allowed to be taken multiple times however for some reason Dwarf's Toughness was not deemed a Fighter feat (the other two were). It may have been a typo or otherwise -=shrug=-
None of the feats required previous feats to take them.
Improved Toughness is definately better than all of the 'X' Toughness feats on the conditions that
1. you only want to spend one feat on hit point improvement
2. you are not willing to combine classes for base saves improvement.
However if you took 2 or 3 classes you could achieve more hps earlier on and survive to those high levels where Improved Toughness is better.
Of course you can argue that you want a single feat to take you to 20th level or what have you ... great ... you win the argument. But if you really want hit points, then a taking just one more HP boosting feat really makes you tougher for most of your career.
The following is true ...
1. Improved Toughness wins at levels 1 and 2 always.
A 2 class combination at 8th level can have +18 hp from 2 Giant's Toughness OR +15 hp if they took a Dwarf's early on (3rd level).
The Dwarf then Giant combination would be better than Improved Toughness until 7th level where it would be 1 hp behind and then it will be +7 ahead at 8th level. At that time you would have to make it to 16th level to regain Improved Toughness' domination.
However an 8th level character with +15 hit points is much tougher than having +8 hit points.
But yes it is 2 feats that you are required to spend on hit points.
D