When dealing with Pokemon, it's good to use a different type of move for each slot; in other words, for different type moves. Although, some people might have more than one STAB move, or any move type in their move slots.
Blaziken can learn: Fire, Fighting, Flying, and Normal attacking moves leveling up.
Without TM, you can definitely have 2 Fire moves in your move slot: Flame Charge and Blaze Kick; while the other 2 can be Sky Uppercut and Brave Bird - normally because most people wouldn't care about slash.
But, you might want to use HM's/TM's to increase move variety, making you limited to one choice - Blaze Kick. Blaze Kick is a much better fire move to go with because your Blaziken can possibly have the hidden ability speed boost; Althought it's mega will definitely have speed boost - making flame charge somewhat pointless.
Blaziken has great speed already, and a lot of attack. It all depends on your battling style. If you want to finish your opponents off quickly, choose Blaze Kick > Flame Charge (a popular battle style). But, if your a strategic player who likes to think about battle - and out-speed the other Pokemon, go with flame charge.
In the end, it's completely your choice. In my opinion, I would choose Blaze Kick because my blaziken knows speed boost, plus it's mega is a definite speed boost (2 things in one move). Your Choice! Hope I helped! :)
