I have a few due to not knowing many servants before playing Fgo but I’ll give the top 3:
First was Altera. Not that I hated her, but at the time I only knew servants from the Fate animations and Extra game and I was hoping for someone I knew, like Mordred or Tamamo. I also didn’t like some elements to her design. Specifically her skinny frame and the red footwear. But she was a 5 star and looking at the odds, I was grateful for my incredible luck. I figured once I got a Saber I wanted she would take a backseat. A year later and she’s the undisputed queen of my servants and I can’t be happier she was one of the first servants I summoned. She’s carried me through some of my most difficult battles (including some that had Archers surprisingly) and while the Septem Singularity feels more like a filler when compared to the others, I enjoyed her last minute appearance and how she’s basically a juggernaut everyone panics over once she arrives.
Eurayle is another. It’s funny because I got her and Stheno in the same roll and initially used Stheno more because she had better stats and could brute force her way through a lot of initial problems. Again, I figured Eurayle would get replaced once I got an archer that I wanted, like Atalanta. Then Euryale fought a male enemy and everything changed. Numbers came up on her NP like she was 4 star in hiding. As time went one Eurayle became a first pick while Stheno got regulated to the background. I like both of them, but when push comes to shove of who gets leveled, Eurayle is first. She makes me forget there was time I wanted Atalanta as one of my Archers.
Finally Blackbeard. I like his design and despite popular opinion, I like his personality and antics. Yet I expected to drop him very quickly. Partially because there were other riders I really wanted, like Drake. Mainly because of his Bronze status. I was hoping the great Blackbeard would at least be a 3 star. His bronze stats didn’t make me think much of him other than someone I use to keep my party cost low. Okeanos Singularity gave me a higher opinion of him with there being more to his otaku character and comedic role. He plays a fool or can get serious when needed and his time in my teams felt the same. I’d put him in a battle expecting him to be one of the first to go down and he usually ended up being one of the final servants standing. He gave out surprising damage, did well in supporting teammates, survived, and at least for me, was draining NP gauges at a decent rate. Recently he’s been outpaced by another unexpected servant I didn’t think I’d like in Kintoki Rider, but he continues to stay one of my first considerations if I need a Rider.