I agree that the Tough feat is the most effective at raising an individual PC’s Hit Point maximum (PHB, p.170). It is the equivalent of a +4 to Constitution, without the benefit to saving throws and other game mechanics such as holding breath.
There are other feats that provide +1 to Constitution:
- Aberrant Dragonmark, ERLW p.52
- Durable, PHB p.166
- Dwarven Fortitude, for dwarves XGE, p.74
- Infernal Constitution, for tieflings XGE, p.75
Thera are also feats that provide you with an option of choosing +1 Constitution:
- Chef, TCE p.79
- Crusher, TCE p.79
- Dragon Fear, for dragonborn XGE p.74
- Dragon Hide, for dragonborn XGE p.74
- Orcish Fury, for half-orcs XGE p.75
- Resilient, PHB p.168
- Second Chance, for halflings XGE p.75
- Skill Expert, TCE p.80
- Tavern Brawler, PHB p.170
It’s also worth pointing out that the feat that can potentially increase the most amount of Hit Points for the greatest number of characters in a party of six is: Inspiring Leader (PHB, p.167)!
A 20th level character with the Tough feat will have an additional +40 hit points.
A 20th level character using Insipiring Leader on a party of six with a Charisma score of 20 (+5), would provide: 25 HPs to each character; so 150 temporary HPs for a party of six.
The characters can benefit from this feat after a short or a long rest. So potentially, in most campaigns where players choose to have 1 long-rest and 2 short-rests. This would mean that each PC could get +75 HPs over a day’s time, and it would be wopping 450 HPs in total if all six party members benefit.
You do need to consider where you pool your ability points though as at lower levels to get the most out of Inspiring Leader the character that uses it needs to have a high Charisma score. This is not always a bad option though as it will have good synergy with the Intimidation skill which if you are proficient in can prevent you from even having to fight in the first place. If you are lucky, you might be able to talk a player with a high Cha score in your group into taking this feat.
Lastly, I think it’s important to note that, because you are a Fighter, it may be worth considering a combination of increasing your maximum or temporary pool of HPs as well as working on getting a high Armour Class. The AC may save your bacon more than a large pool of HPs because the creatures might not even be able to hit you. If you are subject to lots of critical hits it may be worth considering Adamantine Armour as well as this will turn a critical hit into a normal hit (DMG, p.150).