This is very easy for me.

Flash Gordon - all time great, neve gets old, fun to play with peeps or by yourself
Skateball - fun game, great player
Elektra - bottom handful of Ballys for me. Dont like the layout, bottom PF is just boring to me. Looks great and all that but no thanks, I just don't get it

Like all three, owned two of them (Flash Gordon and Skateball).

In order of what I am eager to play if I saw these three next to each other and they are on free play
1)FG - something about the brutality. (I still own it)
2)Skateball - great layout and a lot of fun.
3)Elektra - fun title, but someone has to get last.

If I have to pay for play...FG may only get one or two credits. Games are just so quick most of the time

Love this era. Not hard to tell if you look at my list.

Flash was my first experience with Pinball sadomasochism. Our local Pizzeria had one when I was a kid and let me tell you, most kids stopped begging their parents for quarters to play it right quick. But I could never get enough of Ming abusing me and then inviting me to 'Try Again, Earthling'. Learning how to plunge up a proper serve to the upper flipper was probably my first ever pinball lesson.

Skateball is just straight up Bally bliss to me. I am completely in love with bottom of that machine and if it had 3-4 Skill Shots in it...hell I could put 5 in there without blinking an eye...it would be a top 3 machine for me. I am always trying to make a clean plunge to the outside-inlane and hit the saucer or the spinner. Even without a special award, it's just too much fun to do to not try.
It is always one I will play multiple games on in any lineup I see it in. I will sometimes burn most of my night on it if it is setup well.

Elektra is one I haven't seen since I was a kid and can't say I remember much about it at all. Nothing really about its ruleset or its gameplay.

Layup.

Flash Gordon by far #1.
Skateball 2
Elektra a distant 3.

I had mine for awhile, but it left just like many others. I didn't sell it because I didn't like it, it was just to rotate games out of a limited space. I thought the rules were pretty slick, but not really meant for the casual/newbie player, so I would often find myself trying to explain the rules to guests ("oh, these games have rules?") If you don't pay too much and put too much into it, give it a try. You can always sell an Elektra later, its not an extremely common game so someone is always looking for one of these classic Ballys.

I would only put Elektra at #2 because I prefer the artwork over Skateball, and I am somewhat biased being a former owner.