I felt as though TPS Should have been DLC and as for the best Borderlands in the series I would say it's the very first one they made I especially loved the The Zombie Island of Dr. Ned DLC I hope in the future they make a Borderlands game with the Horror theme all they need to do is look back on the DLC they made and expand it into a full blown game it would be very creative
An idea of mine would be like each character has a unique ability which has a Monster skill tree that ties to it, this would turn you into some sort of Monster but in order to do that you would need to fully upgrade the Monster tree to unlock the ability once unlocked you can transform into the monster such as a Vampire, Werewolf, Zombie, ManBearPig(LOL) now you wouldn't fully take on the appearance or fully act like the Monster you turn into you just get the strengths and maybe weaknesses to balance them out and slightly take on the appearance of it for example..
Can replenish his health from kills and can use bats as minions to attack enemies also gain the chance to "Charm" enemies to make them an ally until they die or your transformation wears off
Has very bad recoil when aiming and bullet spread is widened, but can take massive amounts of damage and deals electric elemental damage regardless if a gun already has another type (if a gun already had electric elemental damage then it is doubled)
These are a few ideas I would like to see in a new borderland game, but I doubt the developers would create an entire new borderlands game revolving around monsters and such but hey I can dream right? Lol If you want to see or have any ideas you want in the next game what would they be?