I would disagree with this having owned all four. I'd rank them as far as ruleset depth:
1) lotr
2) sm
3) st:tng
4) ij
But that isn't to say IJ isn't a deep game, it's got a ton of depth, just not as much as the other games mentioned. And then next, game difficulty:
1) lotr
2) sm
3) ij
4) st:tng
And then which games would I recommend:
1) sm
2) st:tng
3) ij or lotr (toss up)
Spiderman is a great blend of fast gameplay + being pretty difficult (but not impossible). Probably the best overall Stern along with TSPP. St:tng is my favorite Williams games. Great ruleset, fast and flowing shots + the fact that completing modes contributes to the wizard mode score make it a fantastic player. IJ is another great game. I think it probably has the best modes of any game ever made. It's biggest downfall is that when you're outside of modes the shots don't count for much (which is solvable by just starting modes one after another). Lotr is a great game as well, but as you get better at the game the games become super long and repetitive. Offers the best long term challenge, but isn't a game you can play every day.
Honestly you can't really go wrong with any of them, but SM and ST:TNG are a notch above the other two.