I initially thought the 5.8" white-on-blue LCD module would be readable in sunlight, but having flown with it a few times now, the white pixels mostly vanish under direct sunlight (even with the blue back-light set to maximum brightness). For sunlight readable screens:

• Any monochrome LCD should be of FSTN Positive type, preferably with black pixels on a white/grey background.
• Any color TFT LCD screen needs to have an ultra-bright back-light with an absolute minimum brightness of 700 nits (1 nit is equivalent to 1 cd/m2).

For sunlight readable color TFT screens, the transflective variants (which transmit light when the back-light is turned on and otherwise reflect light) are extremely expensive and difficult to procure. TFT modules with ultra-bright back-lights are easier to procure but typically cost a few hundred dollars. After multiple days of googling, browsing catalogs and comparing screens, I've selected the following options, which should suit most cockpits and budgets:

1. Nokia 5110 LCD Module

1.4 inch (29 x 19 mm active area) 84x48 pixel monochrome, (less than $2 on AliExpress).

These modules use the PCD8544 controller with a serial interface to the microcontroller. Their small size makes them ideal to fill a standard 2.25" instrument panel hole. One could have multiple of these in the cockpit, each with enough space to show a graphical widget, 1 to 2 large font parameters, or 6 to 7 small font parameters.

2. EastRising ERC240160FS-1 Module

3.4 inch (72 x 48 mm active area) 240x160 pixel monochrome, (around $11 from BuyDisplay).

The solderable FPC ribbon interface supports both 8-bit parallel as well as serial communication to the ST7586 controller. The screen size, pixel density and price make this module quite attractive for most use cases.

3. Newhaven NHD-7.0-800480EF-ASXN# Screen

7 inch (154 x 86 mm active area) 800x480 pixel color, 1000 cd/m2, (around $80 from Newhaven).

This is by far the cheapest sunlight readable TFT screen that I found in this size category. It uses a 24-bit parallel RGB interface, which is supported by most single board computers (e.g. the RaspPi Display Parallel Interface [DPI] as a multiplexed function on the GPIO pins). Alternatively an external controller can be used to drive it via HDMI. Touchscreen versions are also available, trading off a bit of brightness.