I would think X Flight would be open on Memorial Day weekend. Since it is the newest attraction on the rides side, it will be packed early. If anything, hit Superman first due to the low capacity. Sit in the middle to get the best of both spikes (just enough twist and just enough height). While in the area, also hit Big Dipper, the park's gem. You'll love it! Then go to X Flight if line itsn't too bad.
Afterwards, head over to the western area of the park. Serial Thriller has become rather rough and it is a Vekoma SLC. Ride it if you want to, but if Villain is open by the time you get to that side of the park, do it. Villain has to be the best coaster in the park right now. They replaced the old restraints with the newer, wider ones and makes the ride much better. If you'd like, take a spin on Double Loop just for the ride count. It's pretty tame. If not,head over to Batman Knight Flight.
Opening weekend saw all of the queues full but the line moved fast. Don't be discouraged if the line seems quite long. Definitely sit on an edge seat in the back for great hangtime in the loop. The front seat line is always long but if you want to, sit in the middle over the track to make the floorless effect work.
If you need a break, I highly recommend taking the short stroll over to the wildlife side of the park. It's very, very nice. The Batman Water Stunt Spectacular is IMO quite good and will be even better once they get the pyrotechnics working (I saw a dress rehearsal last weekend and enjoyed it). Grab a bite to eat while over here as there is more variety to choose from than on the rides side. Don't miss Pirate's 4-D which is very entertaining. It's a 3-D movie with extra effects thrown in.
This is just a few suggestions you might consider doing while at the park. Do not miss getting a ride on Villain at night - it's awesome.
X Factor
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SFWoA: An Unofficial Guide
http://sfohio.tripod.com