Anima - 41
Wind - 42
Thunder - 58
Fire - 61
Ice - 28
Dark - 21
Light - 20
Next is a list of what characters are in each affinity.
Anima - Eliwood, Kent, Oswin, Farina, Renault, Canas, Athos
Wind - Lyn, Sain, Fiora, Hawkeye, Matthew, Wil, Priscilla
Thunder - Hector, Wallace, Heath, Bartre, Geitz, Erk, Serra
Fire - Lowen, Vaida, Dorcas, Harken, Guy, Dart, Rebecca, Nino
Ice - Marcus, Raven, Legault, Jaffar, Pent, Nils, Ninian
Dark - Isadora, Karla, Rath, Merlinus
Light - Florina, Karel, Louise, Lucius
You should select the affinity with the largest group of good characters. For that reason, I recommend Fire. All of the characters with the Fire affinity are pretty strong, and three of them get stat bonuses in Hector's Hard Mode.
Below is my opinion on all the affinities.
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Anima - I never pick the Anima affinity because I don't really like any of the characters in it except Canas. Minor side note; the default affinity is Anima. Eliwood normally turns out good, and Oswin is one of the strongest characters in the game. However, I rarely use Kent because my Kents normally get terrible Strength, even though when you calculate his Strength using his grwoth rate, he only has 1 less than Sain as a level 20 Paladin. Farina is statistically tied with Hector as the strongest unit in the game. Renault is a horrible unit though. Canas is another exceptional unit who is one of my favorite characters.
Wind - This affinity has a plethora of very powerful units, with the exception of Matthew, so it is a pretty good choice to pick as well. Lyn is better then Eliwood in stats, and that's saying a lot. Sain's only problem is his low resistence, and that doesn't always bother him. Picking Wind makes Fiora a lot easier to raise. Hawkeye is statistically better then Dart and you get him on the earliest Chapter you can get an Ocean's Seal. Matthew I use only to unlock chests before you get Legault and to steal the Silver Card as well as the Member's Card. Picking Wind makes it easier to do these things without dying. Wil is statistically the strongest Sniper, and he's relatively easy to train as well. Priscilla is tedious to train, and takes forever to get an S-rank in Anima Magic, but she is a strong character.
Thunder - I normally select the Thunder affinity because I always play Hector's Hard Mode, and you have to bring Hector in every chapter, so he has to be good to survive towards the end. Also, Bartre is the strongest Warrior in the game. Erk with the affinity bonus is a stronger Sage then Pent is. Serra is the best Bishop even without the Tactician's Bonus. Heath gets a stat bonus in Hector's Hard Mode. If you raise Wallace to level 20 before raising him, which can only be done in Lyn's Hard mode, then he's as powerful as Oswin. If your Wallace turned out bad, then you can elect to get Geitz instead of him on Four-Fanged Assault and Geitz gets a stat bonus in Hector's Hard Mode.
Fire - This is a great affinity to select. Lowen is the strongest Paladin, statistically speaking. Vaida gets a stat bonus in Hector's Hard Mode and she's pretty good as it is. Dorcas' only problem is that he has around 13 speed as a level 20 Warrior without affinity bonus. Harken gets the largest stat bonus in Hector's Hard Mode out of all the characters that you get and he's pretty good even without that. Guy gets a stat bonus in Hector's Hard Mode and is an exceptionally powerful character as it is as well. Dart's stat holes are fixed with the affinity bonus. Rebecca becomes stronger then Wil with the affinity bonus. Nino is the strongest Sage in the game.
Ice - This is probably not a very good idea, because about half the units in there are bad. Marcus is bad in terms of stats. Raven isn't that bad in terms of stats, but he is outstripped by Harken in Hector's Hard Mode. Legault is an exceptional unit. While Jaffar ties with Karel as my favorite character, I will admit he's not particularly strong in terms of stats. He's not horrible either, mind you. Pent is good in terms of stats, and he's already prepromoted so he's easy to raise. However, he's the only unit besides Legault that you can say that about in terms ofs. Nils and Ninian are horrible units that I never use except in compulsory chapters and to get 19x part 2 in Hector's routes.
Dark - This is arguably the worst affinity to pick. Isadora ties with Marcus in bad stats. Karla, while having good stats, comes VERY late in the game, making it nearly pointless to raise her. Rath is a very good unit, whom I always use. You don't have him for a large portion of the game, but that doesn't really matter. However, he's the only good unit with the Dark affinity. Last is Merlinus, and his dodge is all he needs, and he always gets that high. So not only are they short of units, they only have one good unit.
Light - This is another affinity I would never consider. Florina IS a good unit. Karel IS a good unit. Lucius IS a usable unit. Louise IS a usable unit. However, I use neither Lucius nor Loiuse. I dislike the Sniper class a whole, and Serra is the better Bishop that gets a Tactician's Bonus in my most often selected affinity: Thunder.
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Most players select whatever affinity has more of their favorite characters then the rest. For me, that happens to be Thunder. Anima has 2, Wind has 2, Thunder has 5, Fire has 3, Ice has 2, Dark has 1, Light has 1.