which servant is better?

The one you like most

For rider I think Martha is more useful than Astolfo. Although she hit lower than that trap all her skill is better.

Martha and Fran.
Astolfo's "only" good point for farming is "only" based on his strengthening quest finished 1 year later. Save everything for Martha now.

It is interesting but also sad that gamepress even consider "High Star Absorption Rate" as his strength. OMG, what about other riders?

Martha's normal damage is not good. But her team debuff removal and enemy debuff is almost unique and best. Many challenge quests afterward need that.
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Fran's better than Beowolf because...Role issue. Fran's NP farming work is better than Lancelot or way better than Little cat Tamamo, but besides of her excellent NP damaging (almost best AOE NP, including the class advantage to all) she can also give some support like star generation by NP and debuff skills. Though the demerit effect is a kind of frustrating, but sometimes her duty is just to give a farming NP shot then retreat, leaving the battlefield to 1 SSR DPS with ST NP (class advantaged)+2 supports, not like Lancelot's unreliable role as a critical DPS because both of them are quite fragile, but you don't know when the enemy will crush Lancelot, and you will find it is even more frustrating that when Lancelot generated enough stars but next round there was no card for him, or the enemy just kill him after or even before his NP releasing.

So it is hard to find any Berserkers have same role as Fran, or even other classes. For Beowulf, his NPBB chain is good, but still quite a glass cannon with mediocre NP gain. So far if you didn't maxed Beowulf, then you can give up. Later on like in JP, Fran is still shinning as usual, but no evade only gut made Beowulf almost cannot do anything, not like Herc.

Beowulf has one best male graphic and sprite in FGO. I am quite sick of Vlad Lancer, Ceaser, Spartacus, Darius, or Edison and Ivan. But Beowulf just made me think "oh man, every male servants should just looks like THAT! Or Emiya and Bedivere at least!", so cool remind me as Wolverine. But........Again, if you like them, like a fan in FA for Astolfo, my suggestion is suggestion only.