The Tenser's transformation spell states (emphasis mine):
[...] You can attack twice, instead of once, when you take the Attack action on your turn. You ignore this benefit if you already have a feature, like Extra Attack, that gives you extra attacks. [...]
What features give you extra attacks that aren't the Extra Attack feature itself and thus do not stack with Tenser's transformation?
The description of the Gloom Stalker ranger's Dread Ambusher feature states, in part (XGtE, p. 42):
[...] If you take the Attack action on that turn, you can make one additional weapon attack as part of that action. [...]
On the first turn of combat, a Gloom Stalker ranger would already have an extra attack from Dread Ambusher, so they would not benefit from the extra attack from Tenser's transformation.
The description of the warlock's Thirsting Blade eldritch invocation (for Pact of the Blade warlocks of 5th level or higher) states:
You can attack with your pact weapon twice, instead of once, whenever you take the Attack action on your turn.
Warlocks using Thirsting Blade would already have an extra attack, so they would not benefit from the extra attack from Tenser's transformation.
The Path of the Beast barbarian's Form of the Beast feature lets them manifest a natural weapon, which can take the form of claws (TCoE, p. 24-25):
Claws. Each of your hands transforms into a claw, which you can use as a weapon if it’s empty. It deals 1d6 slashing damage on a hit. Once on each of your turns when you attack with a claw using the Attack action, you can make one additional claw attack as part of the same action.
When using your claws, once per turn you can make an extra attack as part of your Attack action. This feature's benefit would not stack with the additional attack from Tenser's transformation.