It's all up on the other ninja you own or you plan to get. Every main is weak or strong depending on your other ninjas.

If you think to start from zero then the current best option for f2p is to gather 420-440 seal scrolls and get all the jinchuriki from the two jinchuriki treasures, getting sailor sakura when available and going with the azure fang cancerous team (han, roshi, sailor sakura with poison tai and full healing mode). If not you can gather 120-140 scrolls, go in kage and depending on what you get you think your next moves.

In general the han/roshi team while being pretty strong is also quite boring :-), so, really is up on what kind of playstile you like the most. The problem with kage treasure is that you can't at all take for granted that you'll get the ninjas you want, but you have a quite high chance to get a decent lineup by not investing that many scrolls. When i said to gather 120/140 seal scrolls and look at what you get there i meant that you actually change main depending on the outcome. Lets say you get mei and chojuro or mei and mabui or ao among the commons, then you go azure fang and you buy tenten gnw with coupons. Lets say you get danzo and mifune, then you go midnight blade. Lets say you get gaara kk and bee sst then you go earth main and you go for roshi in jinchuriki 1, etc... that's what i meant. Generally speaking within your first 3 kage rares you get a designed road to follow (so within 180-210 scrolls in average), but just after the first two you are well placed (unless are dupes, but is almost impossible).

about the lineup in the meanwhile is mostly up on what main you will select. Azure fang and midnight blade have it easier, i think, in early times.