If you have no specific use for them, then simply use it on one where you want to save some materials on.

-What Otakon said. Look at the requirements of the servants you want in the future, pick the one who dont share the same items

-Enkidu

Enkidu is a tank and his third skill heals all of his health.

I use MHX Alter, she is a good servant that has received multiple rank ups on her skills and NP. So I may be biased to say she is a recommended pick, especially if you have Skadi.

However, I will say ultimately, pick a servant you lack materials to ascend or need to save materials on. Between your options, either Enkidu or MHX Alter.

Enkidu

Enki is super great, like GOD DAMN!!(Yeah @Saciliathan or @NotMalka , im thinking about u)... but i think that for general purposes since shes a berserk, MHXA would be better to use the sa...

1st u have medusa to do iskandar job

2nd in terms of st, MHXA has the berserker advantage to do damage, and with tons of Skadi on support list, it gets easier...

But.. plot twist: u do whatever u want, and whats class advantage on fgo?

Where's my Dumuzid plushie DW?

I mean... One of the four has the sexiest voice that ever was in FGO. And it's golden. And it's fluffy. And it has incredible humor.

Should I go on?

I want him to whisper to my ear Endless battle at nights...

MHXA... why? As if you can't tell from looking at my avatar.