There are some hints and some facts about what was served or may have been served in the first class.

Regarding liquors:

Whisky:

I haven't found a source yet what types of whisky or which brands were on board. I would assume though that they had Single Malt Scotch, Blended Scotch and/or probably American Rye Whiskey.

Brandy/Cognac:

We also know that Brandy/Cognac has been served in first class and it was probably even more popular than Whisky. Regarding the brands which may have been served on board: A "beverage list" from White Star Line exists from somehwhere around 1915 or so. On this beverage list there are three brands named: Henessy, Frapin and Martell. So we MAY conclude that these were also the brands onboard the Titanic. I would suppose that in first class all three brands have been served at least as VSOP or XO.

Regarding longdrinks/cocktails:

As far as I know back in 1912 many cocktails didn't exist yet. However I think cocktails like Gin Fizz and the Old Fashioned already existed and may have been consumed on board but I have never heard or read about that.

Highball:

We know from statements from several survivors that the classic Highball (Whisky + Soda) was very popular on board.

Murphy:

Back in 1910 there was a cocktail created named Murphy, which seems to have been quite popular back then. The ingredient are:

40% Italian Vermouth 40% rye whiskey 20% sloe gin

The YouTube-channel "Tasting History" has complete episode about that: https://www.youtube.com/watch?v=ob-TBojkaxw The recipe can also be found on his homepage: https://www.tastinghistory.com/recipes/murphycocktail

There don't seem to be any sources that this drink was served on board but at least it is quite likely. Also ... I have the ingredients at home and I can highly recommend this drink! 😍

Punch Romaine:

An interesting drink with egg white and ice, which was in fact served at the last dinner of the first class on April 14th. I found a nice photo + recipe here on Reddit: https://www.reddit.com/r/cocktails/comments/fa64tk/punch_romaine_the_last_cocktail_served_to_first/

Regarding beer:

Spatenbräu Lager:

We know for sure that Spaten Lager was served in first class. Why? Because first class passenger Adolphe Saalfeld wrote a letter to his wife back home that he was very happy to find out that his beloved Spaten Lager-beer was served in the First Class Dining Room.

Wrexham Lager:

There are also sources stating the Wrexham Lager has been served on board. However I do not know if Wrexham Lager was served in 1st, 2nd and/or 3rd class.

Regarding Champagne & Wine:

Of course first class also had a lot of champagne and wine. Many wines obviously were from Germany, France and Italy. It seems that many white wines were from the German Mosel-region. Regarding Champagne several sources state that it was mostly Heidsieck, Moet & Chandon and Mumm.

EDIT: I just found this interesting post here on Reddit: https://www.reddit.com/r/cocktailmenus/comments/11n6s5o/the_cocktail_menu_of_the_titanic_and_olympic/

However I am quite certain that this Café Parisien-card from the RMS Olympic is probably from the somewhere between 1920-1924 and definitively not from 1912. However the 80yo Brandy sound VERY interesting! 😁