Who should i roll for

Since you like all of them, in a gameplay sense you could lean on saving up everything you can for Merlin. Once you have Merlin you can breeze through most of the game since you can pair him up with almost anybody and turn them into an unstoppable force.

Though Tamamo no Mae actually fits better on your current lineup of servants you listed down since most of them are Arts-based. However Tamamo no Mae isn't limited so she can spook you anytime on any banner.

Also Caster Gil is a 4* and also non-limited, so he can also spook you anytime.

http://fategrandorder.wikia.com/wiki/10M_Downloads_Campaign#SR%20Servant

You will get to choose a single 4* for free by the next September (aside from the limited ones, like Summer Servants, Welfares...).

Caster Gil is in, so, supposing he has not spooked you (and he has about a year of time to do that, after all, non-limited and non-story locked), you can pick him.

Merlin. Always Merlin. Merlin forever. Merlin breaks the game. He's about as easy to use as waver:
1) Use him
2) Press skill buttons(with a little more strategy than waver)
3)Press red cards
4)Press np card
5) repeat steps three and four
6)Win