depends what you aim for, maybe a good p1 ninja. So Madara etr

For F2P, there are amazing ninjas that can be used as a Pos1 Carry like: Choji GNW 4* (Not as good as the others but one of the most accessible units in the game to 4*), Hiruzen Edo Tensei, Kimimaro Halloween, Kisame Samehada Fusion 4* (Not as versable without his BT), Nagato Edo Tensei 4* (Mainly used a support ninja but could potentially be a good Pos1 carry with BT), Kushina Habanero 4* (She's fine but her damage scales greater with BT), Madara Edo Tensei Release 4* (Still good without BT but greatly increases overall damage and sustainability), Kakuzu Earth Grudge Fear 4* (Not really on par when compared to other Pos1 Carries without BT), Minato Edo Tensei 4* (Still good without BT), Six Path Sage Naruto 4* (Having his BT makes him amazing), Sasuke RinneSharingan 4* (Not really great without BT but he still works), and Shisui Kimono 4* (Fine without BT). Hope this helps you decide on which unit to get to become your Pos1 Carry

Edit: You should save for 30K coupons and use it when there is a Fuku event active (I believe normally they have Lucky Stars, Great Plates, Fateful Choice, and/or Great Tree) which are one of the better events to spend your coupons on. This gives you the opportunity to 4* one of the following units above if the Fukumaru Pack includes the Pos1 ninja you are saving for.

He is a very powerful ninja if you can get his BTs. The amount of ninjutsu and attack percent boost scaling is incredible and can easily beat opponents with ~50K =/+ with the right comp.

This is one of the good comps that can pair well with Gengetsu as his damage output is increased by Itachi, Earth Main, and Sasori