EMIYA or Heracles

Heracles is indeed better than Lancelot but they have fairly different roles and the gap isn't that big Heracles is great at just never dying while Lancelot is a God tier farming servent and a crit monster. Overall Heracles is pretty indisputably stronger than Emiya especially at the moment since Emiya doesn't have his interludes to that let him crit for ridiculous damage yet.

Well to make a simple comparison to what you already have take Heracles if you want a bigger harder hitting version of Cu or Emiya if you want what will eventually be essentially a Archer class equivalent to Nursery Rhyme.