Jonin Minato is a pretty good ninja, a lot of people still use him and from what I can tell he still excels in certain areas. Since his price is only around 10k (Could be higher or lower depending on what event you try to get him in), he would be a good option and keeps it in reasonable price ranges for f2p. Another ninja is Shisui Uchiha (I would actually prefer him over Minato), even better if you have anbu itachi. Shisui also costs about 10k coupons, and anbu itachi (Assuming you have 0 fragments of him) would cost about 3k cps to piece together fully, either way, I believe it's best to go for these 2 ninjas if your staying full f2p (Saving your coupons for a expensive event ninja is also something to keep in mind). It's your choice on what to spend on.