I seek a perfect mix between both, I think you can't think of it just as defensive or offensive in a vacuum.
For a pin to be perfectly challenging for me following conditions must be satisfied
1. It has to have enough physical shots on the playfield that matter in progression of the game. That makes substance of what is there to do in that particular game. Your topic has started me thinking about a metric theory of comparing pinball games.
Physical shots present on practically every game are: left loop, left ramp, right ramp, right loop, some lock, hole or saucer. most rules are based so you have to shoot those regularly. Some pins have so much more, some are poorer in design.
Their existence builds up offensive part of the game and their placement and geometry punishing bad shots with drains builds up defensive part of the game. but only if you have to shoot them to accomplish anything meaningful. for instance: bookcase in TAF is integral part of the game, but left side greed targets are irrelevant, redundant and nobody needs to shoot them to progress. No way out hole on Whitewater is integral part of the game in locks and getting on the upper playfield and spotting hazards, but bigfoot hotfoot targets are just sideshow, not integral to gameplay and you ignore them letting them collect bricked shots and ricochets.
I tend to value complexity in games that have many things to shoot for. I get bored easily. Analyzing I came to conclusion that many games have up to 9-11 things to do in general of which 5 or 6 are important. I don't mind chopping some wood as long as there is wood to be chopped. Fish Tales dissapointed me in not having much meaningful stuff to do in that beautiful art package that it offers.
here are some examples:
Bram Stoker Dracula has only 4 integral shots in it: castle loop, tunnel, coffin ramp and lane which mist ball traverses. you collect locks in these shots, start multiballs, collect jackpots and stack jackpots in them, collect awards, collect bats. It also has 6 auxiliary shots that add to the experience but you can avoid those and still play a normal game: skill shot switch, rollovers, bumpers, video loop, altar (because of rats), center target (get castle lock while ball saver is on). I don't count inlanes as you don't shoot for those. there are also two completely irrelevant shots just for bricked shots and ricochets - asylum and left targets. Total of 10 playable + 2 irrelevant = 12
Addams family on the other hand has 6 integral shots: chair, thing ramp (which doubles as thing eject since it leads to that), bear ramp, side ramp, bookcase, vault (slightly off and not same as bookcase). it offers 6 sidegame shots: swamp, trainwreck target, thing eject skill shot (you can use center ramp for this ingame), right loop, cousin it target (only used in one mode), left loop through bumpers. Total of 12 playable shots.
Shadow has 9 integral shots: left ramp, right ramp, inner loop, battlefield shot, battlefield paddle (its unique but lets count it as a one shot to be played), sanctum, khan saucer, start mode saucer, left loop. 2 shots are auxiliary: right loop (not a jackpot shot, not a setup for inner loop and you can collect mongol on the left loop), diverters and inlanes(lets count them as one thing, and theoretically you can play a game without engaging them and randomly finish khan for that 1 scarf requirement), mongol targets and special mongol target are irrelevant, basically just brick shot. Total of 11 playable things.
Congo has 7 integral shots that make gameplay: left ramp, gray lane, left loop, MAP saucer, MYSTERY saucer (video mode and diamont hunt multiball access), gray gorilla lower playfield (again unique counted as one thing), zinj rollovers to light mystery. Other 7 shots are: volcano ramp (IMHO fault in rules not requiring you to have to shoot jackpot here), mine shaft, amy rollovers, satellite targets, perimeter target, skill shot targets (also skill fire), left outlane (skill shot and skill fire). Hippo lane is random and unshootable. Total of 14 things to do.
Indiana Jones has 8 integral shots: mode saucer, path of adventure playfield, left ramp, right ramp, left loop, right loop, ENT target set (doubling function as a lock), captive ball. other shots include bumpers and rollovers. Im probably wrong, but I see rest of target banks on the side same as BSD left target bank - something to be completed by accident. Many modes are packed in those loops and ramps. 10 things to shoot for.
2. It has to have rules that integrate shots existing on the playfield into goals of playing. Scoring in Dr. Who has sonic boom loop having so much more impact on score than other things that it turns it into 1 trick pony. all other things in that game become less relevant and degrade both pin complexity and its monetary value. they all started (pins in general in the 90s) as, what, like $3999 machines when they were new and some of them skyrocketed like MM, TZ and some dropped to be cheaper B/C list fodder like DW.
Good rules improve offensive part of the game and making players shoot more for dangerous shots improve defensive part as well.
Good rules should be both complex - having many things progress into different stages, and to be good in using shots right, for example BSD makes best usage of limited number of shots with great rules and stacking.
I like recent Stern rules, meaning I like Lyman and Keith rules. Out of my price range though.
3. Scoring has to be balanced not to push players to chase only one strategy with most reward vs risk. Good example: the shadow, congo everything scores similarly, AC/DC - lots of different strategies. bad example: terminator 2 - every one chases only multiballs ignoring the rest of that beautiful playfield.
4. preferably (but it is rare) game has to have some non-linearity meaning not having to pursue same goals in same order every time.
sorry for a long post. interesting topic. thinking about it all the time.