What is the best alternative to Protractor+ on iPhone?

Bubble Level for iPhone, S tier, #1 in angle measurement.

Is there a free alternative to Protractor+?

Yes: Bubble Level for iPhone (S tier, #1) is free to download.

Yes. 2 apps rank above it in angle measurement: Bubble Level for iPhone (#1), Measure Angles - Bubble Level (#2).

Protractor+ is A tier, #3 of 6 in angle measurement. Here is everything that competes with it on iPhone, in tier order.

1
Bubble Level for iPhoneShigher tier
Free download · ★ 4.6 · 111,713 ratings · score 92
Utility app offering a bubble level, angle and flatness checks, metal and wire detection, inclinometer, AR ruler, room…
2
Measure Angles - Bubble LevelA
Free download · ★ 4.2 · 2,020 ratings · score 84
Utility app with a bubble level, digital protractor, ruler, compass, altimeter and other measurement tools.
4
Bubble Level Plus+C
Free + IAP · ★ 4.5 · 15,358 ratings · score 68
Bubble level and angle and tilt meter for checking whether surfaces are level and measuring angles and slopes.
5
Pocket Bubble Level XXLC
Free download · ★ 4.8 · 16,952 ratings · score 66
Phone-based bubble level and inclinometer that shows level and inclination angles in each plane, with no internet.
6
Bubble Level Tool - DigitalD
Free + IAP · ★ 4.4 · 2,491 ratings · score 57
Shows your device's current angle, with reset and lock options, for portrait, landscape and surface level use.

What is the best alternative to Protractor+ on iPhone?: Bubble Level for iPhone, S tier, #1 in angle measurement.
Is there a free alternative to Protractor+?: Yes: Bubble Level for iPhone (S tier, #1) is free to download.
Does anything rank above Protractor+?: Yes. 2 apps rank above it in angle measurement: Bubble Level for iPhone (#1), Measure Angles - Bubble Level (#2).