I use 4-3-1-2 mostly. The reason is that when I took over my team, there are players that fit 4-3-1-2 very well, so I kinda kept it as my main lineup for now 😄 Also, I have switched it to some other lineups when I had many injured players or wanted to play more defensively in some specific match.
As Marko said, depends on your players - the best ones need to play, even if it is 4-2-4 in the end.
I prefer 4-4-2 but formations with LW/CAM/RW can be good as people undervalue those positions on the market. You can buy nice LW for 5m but useful LM will be like 10. Same goes for CAM, you can buy top notch player for 10-12m while for that money you can only buy some decent CM.
Spend on CB, save on GK, dont play with 5 in a line or two CAMs (just a theory), you should be good with almost any formation. Just not those weird diamond ones, I don't like them but that is just me.
It's not planned yet to be visually displayed on the match page, just by order, but it is already included into long expected Manual update. There will be visual of every position.