Just saying "best Lancers" is pretty vague so I'll just divide them into categories I could name. (Unreleashed Servants are also included)
Solo potential: Enkidu (VS Divine), Scathach, Cu, Taiga
Farming: Lalturia (Original and Alter), Karna, Eresh, Bradamante
Burst Damage: Scathach (VS Undead and Divine), Brynhild (VS "Loved One"), Tamamo (VS Male), Li Shuwen, Taiga
Semi-Support: Eresh, Raikou
Indeed, each lancer serves their own function. Some might say that Cu Chulainn is the best lancer due to his sheer survivability even though he's only a 3-star servant. Artoria (Lancer Alter), Jeanne Santa Lily, Enkidu, Scathach, Pavarti, Houzouin, and Ereshkigal are all very solid lancers as well, each for different reasons.
Clearly you've never seen me wield my pole, if you know what I mean ;)
I'll show you best solo potential, burst and support...
No, it depend on the situations. With good support and gameplay tactics you can make any Lancers into a hard hitter.
Solo and Harder contents : OG Chu, Enkidu, Vlad III and Ereshkigal.
Crit and Burst damage : Karna, Taiga Fujimura, Proto Chu, Raikou, Scathach, Brynhildr, Tamamo, Lancer Alter and Li Shuwen.
Semi-support : Ereshkigal and Brynhildr and Vlad III.
Farmer and Wave clearer : Lartoria, Lancer Alter, Fionn, Ereshkigal and Karna.
Special damage vs specific enemies : Scathach, Brynhildr, Vlad III, Karna, Tamamo, Enkidu, Proto Chu.
Tank and taunter : Vlad III.
They fill different jobs, Lancer with multiple roles mostly is good but not the best. Their Overall performance on the battlefield will determine they're one of the best Lancer or not at all.
Famous Lancer doesn't mean their gameplay is better than the non-famous one. You can even slay most Archer Bosses with ease by using Proto Chu or Taiga with some support.
Don't forget about Lancer Bradamante and Hozoin Ishun, Bradamante is an NP spammer and wave clearer and Hozoin is a Crit Burst damager plus Solo Lancer.
Leonidas is also a good Buster tank and semi-support. Let me remind this, it depend on the situations.
