I never tell people who to play but if we already have 3 ads with a support and you are last picking jungle (weird scenario but w/e) I would mention please go AP. I get people play their champs but it really pisses me off when we have 3 squishy ads with a sqiushy support and then someone goes like squishy ad top or squishy ad jungle. Like I get a lot of people are one trick ponies but I feel you need to have some common sense or skill to play a few champions.
Now... people will tell you exactly what champ to play top or mid for example vs the enemy. I don't agree because although xxx champ might "counter" the enemy if you go that champ just because its a "counter" and don't know how to play that champ you will probably end up doing poorly.
I get where you are coming from, and I personally never really say anything to people. Just saying.... sometimes people really would like players to go a certain type of champ and its frustrating when they go their one trick pony champ that does not help the team at all.
I have played enough ranked though where I have seen some weird comps where you think "yep... no way we win this one" and you end up crushing the enemy and other games where you think "damn no way we going to lose this look at our team!" and get rofl stomped. Usually its best not to say anything.