If it was entirely down to the SW I like least, I'd vote for The Smiler.
However, I accept that The Smiler is popular and hard to replace. It has a Nemesis-type factor about it in that it's both iconic and likely quite hard to replace with anything more exciting. I'd say that The Smiler is perhaps the park's most popular ride at present alongside Wicker Man; people may speak about Nemesis, but I don't think it is/was the park's most popular ride, whereas I think Smiler could stake that particular claim quite solidly.
So if I were combining both my own personal preference and practicality-based factors, I'd pick Galactica. It's my 2nd to least favourite SW, it takes up a huge area (the largest of any coaster on park, I believe), it's not that popular or iconic (as I said before, the fact that they rethemed it and tried the VR speaks volumes about its popularity and iconicity, in my view), and things like the restraint sensors do appear to be growing somewhat unreliable as the ride gets older.
I do feel that Spinball and/or Rita will probably be removed before any of the current SWs, though.
As for Oblivion, I could see it being retracked. A potential retrack job would be cheaper than Nemesis, and purely based on practicality, I think it would be a similarly tough ride to replace. On the flip side, however, I don't think that it's one of the park's more popular rides nowadays, and I also think that while it was very popular in the 1990s, it's rightly or wrongly seen as somewhat second tier within the modern day lineup. It pains me to say it, because I absolutely love Oblivion, but I do wonder if it is popular enough, iconic enough or remarkable enough to be deemed worth saving.