Gacha

I take it as you mean new servants only, or the rest can be found in the tier list page. But as far as I know, there isn't any servant that is as broken as Merlin, to the point where the game has to adapt itself to nerf him indirectly.

There are still many strong servant from each class next year though, but if you need something that is more universally useful you should aim for supports like Skadi (only available at around 2020). She is the more balanced, Quick version of Merlin, and you pretty much need her to make most of the Quick based Servants to work properly. Like Merlin making all Buster Servants OP now, Skadi raised the viability of all Quick servants single-handedly.

Personal honorable mentions:- King Hassan, Summer Nero, Summer BB.

Or you can look up to the upcoming servants and determine which one you like most or need the most.

The best banners next year are, imo, chaldea boys 2019 and fate extra ccc. The first has Waver and Cu alter on rate up on different days, both being among the best at what they do and universally good. The only downside is they both share their rate ups with proto Arthur, who's kinda mediocre. The second has many servants on rate up on different days, and the one I'm recommending is Gilgamesh, though Nero bride and Tamamo are both good too. Aside from that, fate extra ccc also has a few good CEs on rate up, the best being a fragment of 2030. It's a good chance to go for both.

EDIT: RIP Brynhildr's not in ccc :(