The following is how I pick a Free SSR for Fate/Grand Order from FGO Festival 2023 ~6th Anniversary~. I will go into how I break up and reduce the list of 33 to 1 I ultimately pick.
Optimal Selection Phase
First, you should create a list of classes of characters you lack the most. As a rule of thumb, you would want one single target noble phantasm and one AOE noble phantasm for each class. The reason for this is so that you would have at least one character that you can easily use to target bosses and another that take out small mobs with their weakness. It is ok to only have one of each type of class but having both would speed up the battles.
Fighting a boss with AOE NP could increase the length of the battle or at worst you will not hit the window to beat the boss without wiping the whole time. While having a single target NP would just make taking down single mobs take a while as you cannot clear 3 of them during single NP. Another issue is if there is a boss that spawns mobs, it would be easier with an AOE servant than a single target one. Doing this will help balance out your roster for any issues that may arise.
If you are not sure about where to Waver "Zhuge Liang (Lord El-Melloi II)", I would always put him at the top of the list. He will always be useful to any player if they do not already have him. For the extra classes I would add them to the list in the following order Jeanne d'Arc then Sitonai then last of all Jinako Carigiri (Great Stone Statue God).
Like or Waifu Selection Phase
Personally, I always have some personal preferences when I pick or roll a character as I prefer if I like the character over if the character is meta. There are a few expectations to this like super meta caster servants of Fate like Merlin, Waver, Artoria Caster, and Scáthach-Skaði. So normally this is the second stage where I eliminate most of the characters I do not have an interest in. As I think it is more important to have characters you like to play with than ones you do not.
The character I like is Altria Pendragon, Mordred, Altria Pendragon (Lancer), Francis Drake, Tamamo no Mae, Nightingale, and Jeanne d'Arc. The reason for Altria Pendragon, Mordred, and Jeanne d'Arc is probably a combination of the animes, personality, and game stories. Altria is one of the key reasons why I got the game in the first place. Francis Drake was because of the pirate theme, her personality, and her role in Sealed Ends of the Four Seas: Okeanos. I like Tamamo no Mae personality and character design. Nightingale is probably the weakest one of my likes on this list as I do like her character design but everything else is not as strong as my other choices.
Noble Phantasm 2 Or Higher
Now I try to weigh out if it is worth if NP2 or higher is worth it. Generally, I like more characters but at times it is more worth having NP2 of a character that can make them much stronger.
The biggest damage jump is normally NP2. For example, Francis Drake' NP Rank A+ at NP1 is 400% but at NP2 is 500% while NP3 is only 550%. Another example is Quetzalcoatl which is a single target NP that has a 200% increase in NP2 from NP1. Most servants follow this growth model so it would make servants stronger with NP2 than NP1 but worth reducing after NP2. Also, the growth matters if it is AOE or Single Target.
Also at times just having NP2 of character will make them viable to farm loop during lotto events as some require damage levels at a certain point to clear the wave. In some cases, Mordred would be worth the pick for NP2 for looping.
Finalizing My Choice
At this point, my list is pretty small after going through the above process. In the end, it was just between Altria Pendragon (Lancer) and Florence Nightingale. Altria Pendragon (Lancer) does a lot of AOE damage, self-damage buffs, Ignores Invincibility on NP, and has 50% NP self-change which is very good. While Florence Nightingale has healing, debuff removal, enemy attack reduction, and buster damage increase. I would probably only use Florence when it is required while Altria I would probably just use random and more often. The reason for that is Florence is more of a buff for only a few servants and debuff removing while I can Altria for farming, fighting bosses, and dealing with things that just require damage lancer. On the like/waifu tier list, I would rate Altria higher. So ultimately I picked Altria Pendragon (Lancer) as my free SSR.
Conclusion
My method involves breaking up the list into needs and then taking out characters you do not like. Then remove characters if you do not need their NP2 if you already have them. The importance is balancing out my account along with picking a servant that I would enjoy having. I use a similar method when picking guaranteed summon roll.
