Elektra is it but it is pre-alpha-numeric. Look up the ruleset... I wrote a 15 paragraph explanation of the rules once and it is NOT complete. As others have said, "deep or complex" does not always equate to "good". Elektra is not just deep, it is very specific. Not for the faint of reading.

Also an early one not yet mentioned is Williams Defender.

Not sure what you mean by "interesting". It's just as "interesting" as Metallica or AC/DC.

My original post was over in the "Elektra Club" thread. There could be errors and others have added a few rules that I forgot about.

It's only about 80% complete but I cleaned them up a bit more today when I posted this:

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I have attempted to compile a list of rules for Elektra. However, by the time I get to the 20th rule, I forget all the others and my head starts to hurt... It does have the most overly complicated rule-set up to that point in pinball history I have ever encountered.
I am now attempting to do this since there is an official Elektra thread...
I know things are missing but here is a start:

LOCKING BALLS
Making green 1-2-3 targets lites left rollover button lites. These lights alternate the direction that they flow every 5 seconds.
Ball rolling over lit buttons in direction of travelling lights around the LEFT ROLLOVER ORBIT, or by dropping 2X-3X-5X "in-line" drop targets behind the captive ball will lite the top 4-5-6 target arrows, one at a time.
Making 4-5-6 targets lites the top saucer arrow to allow for locking a ball and also lites the top right rollover button intermittently to score special.
You can light the green arrow LOWER LOCK by completing ANY set of standup targets (including the easy to hit center red targets) 5 (need to confirm this?) times.
Making the 5 blue standup targets:
Lites left outlane for 10 Elektra units and bottom saucer green arrow for LOCK ball.
Note: You can "lane change" the needed blue targets with the left flipper at any time.

Capturing balls in both the upper and lower LOCKS starts mulit-ball play.

Also, with 1 ball locked in either captive hole, completing ANY set of standup targets will start multiball play.

OTHER STUFF
3 red center stand up target awards:
1st time opens right ball return gate.
2nd time lites ball return arrow to collect bonus.
3rd time lites 2 blue target lites.
4th time lites 2 additional blue target lites.
5th time lites outlane specials, left outlane for 10 Elektra units and bottom saucer arrow for LOCK ball.
Completing 3 middle level targets? don't understand what this rule explains?
Lighting orbit spinners? not sure how or how much they are worth
Captive ball drop target multipliers? not sure to which bonus these are applied.

OBTAIN SOME ELEKTRA UNITS
There a many ways. Here are some:
Completeing any bank of standup targets awards 2 Elektra units. (rules setting)
Making 5 blue targets:
Lites left outlane for 10 Elektra units and bottom saucer arrow for lock ball.
Note: The blue target bank becomes more and more difficult to complete. The targets required to complete the blue bank will flash and will lite solid when collected. They change with each completed set.
Note: You can "lane change" the needed blue targets with the left flipper at any time.

Making either 1-2-3 in sequence or the 4-5-6 targets flashes 2X target for 10? seconds.
Target must be hit when flashing in order to collect double bonus.
Locking both balls awards an extra ball. (rule setting) Maximum of 1 extra ball per ball in play (rule setting)
Striking the red target at the end of the captive ball hallway awards ? Elektra units.

EXTENDED END OF BALL PLAY (LOWER PLAYFIELD)
Mini playfield becomes active after ball-in-play enters out-hole and a minimum of (rule setting;default=6) Elektra units are achieved.

LOWER PLAYFIELD RULES
Each ball you play in Elektra can potentially be extended to the lower playfield.
Each extended ball in the lower playfield "starts fresh" and no progress "carries over" from previous extended ball play.
Your main objective is to actually NOT just hit everything or JUST keeping the ball alive.
The Elektra units translate to the number of seconds you have "unlimited balls" while playing the lower playfield. Once all Elektra units have expired, it is sudden death and either the next ball begins or, the game ends if it's the last ball in play.
What you want to focus on is to hit the 6 orange side targets but not necessarily complete them. The reason is because striking them once lights them but they all go out after you lite them all. Striking a LIT orange target on the lower playfield awards the most points (5,000). Just lighting a target scores much fewer points (need amount here).
Lighting 5 orange targets lites the left orbit arrow.
Completing all 6 side orange targets one time flashes the center green target for 2X and hitting that green target lites it solid and collects it. 2X is the highest multiplier on the lower playfield.
Lighting 12 orange target lites the right orbit arrow.
The lower playfield does have a bonus which increases with each target bank completion and that is displayed in the center. I believe it has a max but I don't know the amount.

Tilt penalty – Ball in play.