Here's my theory about the final member for final boss (entering Gregminster)
I see many people will take Flik-Alen-Grenseal or The Elves unites, and Pesmerga or Crowley or Luc or Sonya because their own status, or Kai because helps you a lot in random battles. But I like to take people who's make sense according to the stories, rather than personal bias or their power/skill/unites...
Here are all people you can take to Gregminster :
I will eliminate all the Scarlet General, since the 4 of them show up at the dialogue with Barbarossa
Sonya, Milich, Kasim, Kwanda
Then i'll eliminate all the optional recruitment which you can recruit or not (because they wouldn't be at the "official" final battle if they only optional stars)
Mace, Sheena, Meg, Gon, Kage, Quincy, Rubi, Lotte, Mina, Clive, Crowley, Kessler, Lorelai, Lester, Moose, Maas, Fuma, Anji, Meese, Sarah, Blackman, Sansuke, Kanak, Kai, Antonio, Hellion, Pesmerga, Sergei, Leonardo, Kirke, Kreutz, Eikei, Fu Su Lu, Morgan
Some people will automatically joins you for some purpose, so i will exclude them, because their part for the war may "only" for that
Tai Ho, Yam Koo : For sail to Toran Castle
Pahn : For battle against Teo
Liukan : To save the Dragons
Kirkis, Kuromimi, Sylvina, Stallion : Saving Great Forest area from Kwanda's Burning Mirror
Gen, Kamandol : For cross the rapids
Griffith : To enter Moravia Castle
Hix, Tengaar : To fill Viktor's absent while he's go to NorthWindow
Mose, Ronnie Bell : To retrieve the Fire Spears for defeating Teo's Cavalry
Krin : To enter Lepant's mansion
Kimberly : For making a fake order to enter Soniere Prison
Fukien : To retrieve the Star Dragon Sword for defeating Neclord
Futch : Ordered by Joshua since he's abandoned from the Dragon's Fortress
Eileen, Juppo : Just follow Lepant wherever he goes
Camille : Join the Army because Gremio owed her money
Kasumi : She comes to Toran asking for help because Rokkaku destruction by Teo's forces
Valeria : Volunteered to fight against the will of her general, Kwanda
Varkas and Sydonia : As a bandit leader and the border guard
We know that Viktor and Flik is a fixed member alongside with Tir, so they are already on the list.
There are Windspun Armor and Hornet Helmet from Gregminster's Treasure Chest, so there must be someone who able to use that for in your party, why would that equipment at the final dungeon chest if no one that you suppose to bring can't wear it.
From the list above only Humphrey able to wear both of them, and Alen, Cleo, Grenseal, Milia can use Hornet Helmet...
So in my opinion, Humphrey must be the final party, and the fact that he is also the original member of the Odessa's Liberation Army makes him a perfect option to fill the first spot alongside with the other original member Flik and Viktor.
We already have 3 S-Range in party. I don't think another S-Range will be fit in "official" final party, even we can put Alen and Grenseal at the backline and use unite with Flik, but it doesn't seems right for me to have 5 S-Range at the party, so i will eliminate the S-Range from the rest (Lepant, Alen, Grenseal). Luc is staff-magic user and people usually put him on back line.
So the final 2 spot for the final party are : Cleo L, Gremio M, Luc S*, Milia M, Warren M
Let's review their "role" on S1..
Cleo : As Tir's guardian as Leknaat said at Magical Islands at the beginning of the game
Gremio : Follow Tir wherever he go, "Revived" by Leknaat just before the final battle
Luc : Leknaat's apprentice. Join the army by Leknaat's command
Milia : Join the army after Tir and Liukan helps to cure the dragon
Warren : A nobleman rescued from Moravia alongside with Viktor
It's time to decide who is the 2 out of 5 from the list... i will eliminate Milia and Warren because their part was not strong enough to be the "official" final party... that leave us with Cleo, Gremio and Luc.
These 3 are strong option since Leknaat is the one who guide Tir from the beginning until the end of S1, and those 3 have a role connected with Leknaat's command. 2 out of 3, pretty much easier to pick rather than 3 out of 73
So in conclusion the final party for my Suikoden I : Viktor - Humphrey - Flik - Gremio - Tir - Cleo