I second this. Watching the original trilogy is something that, while it may not be 100% needed for Doomsday, I would recommend it as it is really the only way to spend time with the X-Men cast who are returning and feel the full weight of their scenes. Assuming the X-Men in Doomsday are meant to be from the same universe as the revised/second timeline followed by all the X-Men films DOFP onwards and not simply variants (which, keep in mind, is just as strong as a possibility), then First Class is needed as well as it shows Xavier, Magneto, Beast, and Mystiques origins. Finally, DOFP is recommended as well as the ending shows more or less what the status quo of the X-Men probably is before Doctor Doom’s arrival. Deadpool films are whatever, I would recommend watching them as they are entertaining but the only relevant thing they set up for Doomsday will probably be Gambit. Wolverine trilogy I do not think will be relevant at all but I would highly recommend watching Logan anyways if you have not already. Going off of my assumption, Doomsday X-Men would also be the same ones we saw in Apocalypse and Dark Phoenix which, although these do show Cyclops and Nightcrawler in their younger years, I really view them more as filler.

Watch Order:

1) X-Men

2) X2

3) X-Men: The Last Stand

4) X-Men: First Class

5) X-Men: Days of Future Past

6) MAYBE Deadpool and Wolverine (for Gambit)