What Rank Was The Most Challenging But Rewarding?

In the Marine Corps it seemed that the enlisted rank expectations were set up to alternate in how much they could suck.
Pvt and PFC - yep, no question, sucked
LCpl – you weren’t the lowest and generally responsible for nothing
Cpl – Now you are an NCO and expected to lead all your drinking buddies
Sgt – NCO, but separated yourself, more experienced, higher regard
SSgt – Awful. Now you have to be a SNCO and do SNCO stuff, but can barely spell SNCO
Gunny – Plenty of responsibility, but plenty of influence and power
MSgt / 1st Sgt – Tired, just tired.
MGySgt / Sgt Maj – Not crap taken from any direction.

sergeant was the most rewarding and hard, had to go before a board

I served as a first lieutenant in the Vietnam War and guided my platoon through enemy fire, friendly fire, and the use of C-4 from a claymore mine to "fire up" the morning coffee in the bush. None of my troops were killed even though five were wounded during one episode some 50 miles outside of Saigon in 1970.

Seaman recruit

Private was the most challenging rank and maybe the most rewarding. Being a private you are the low man on the totem pole. You must complete all the tasks that higher ranked soldiers don't want to do. You are always at the end of the formation and seem to be overlooked by most officers. It was also the most rewarding rank because you learn so much. You learn the basic soldiering skills needed to survive in military operations. It is also such a great accomplishment and honor when you get your PVT2 rank.

All of them!

All ranks are both challenging and rewarding! It's what you make and do with the situation you are in. Regardless of the position you are in or the command climate, it really boils down to the individuals decision of how they will follow or lead.

Sgt E5

For me it was Staff Seargent. Transitioning to this rank was challenging because it was the first time I had to draw the line between business and friendship. What I mean is, I had to often lead those whom I was friends with in situations that caused friction with my friendships because of my leadership style, so that I as the leader and we as a team could accomplish our task. The rewarding part came when my, peers and leadership acknowledge my ability to lead by nominating me for awards and giving more opportunities to lead in greater tasks proving that they trust my leadership capabilities.

The most challenging but rewarding rank was being converted from a Spc-5 to Sergeant E-5. There was a lot more responsibility and growth required. I was no longer a technician but took on the responsibility of being a squad leader. This prepared me for the challenges that would come with my promotion to E-6.