Which of the Sniper Elite games are worth playing?

They're all mediocre.

I wouldn't call them "stealth" games per say since its not a primary element but I have Sniper V2 to 4 because a friend and I would always play coop for them.

Games and mechanics honestly dont change much between the games, just locations and fidelity a little bit. 4 became a little bit more challenging trying to snip gas caps off escort boats patrolling the shores.

the campaigns/missions are pretty short, like 6-8 hours short. So you can get V2 and see if you like it and then get the other ones.