Software

Photoshop:

I like photoshop to animate because it's intuitive for me and I can do quite details drawings. It has many shortfalls though

Animate:

Animate does onion skinning and 'tagged' colours, where the colour changes across he whole animation if you change it in one frame

OpenToonz:

I really liked the idea of OpenToonz, but the user interface is dificult to navigate. I find it has a steeper learning curve than most and for the level on animation I'm working on right now, it has unnecessary steps to take just to animate, such as having a seperate drawing board from the animation board

TV paint:

It seems TV paint is a good combination of usability and technical proficiency. I find it harder to find decent tutorials on it but once I get the hang of it I think this is the best one for me. I like the adjustments you can make to the onion skinning and how easy it is to colour
Basics
Colouring
Interface basics

Aaron Blaise, getting into some more useful tutorials