All three are winners. It seems rare that someone posts one of these and then lists 3 fantastic games. So you won't go wrong with whichever you pick.
So then on to your question, which is the best game to get. And that depends on what you're looking for.
LOTR is a quest. Over time the points mean less and less as you learn how to destroy the ring (DTR). Yes, your ball time will get longer, but you will get farther as well. Once you've destroyed the ring you start after There And Back Again (TABA). Then you are really trying to complete the multiballs (very tough, but the super jackpot calls are great!). And then you are supposed to combine all of those to get to Valinor? Good luck with that. I really miss my copy of the game, but I'll own it again. It is just a great quest type of game. So deep, and so brilliantly done.
Tron is a slap in the face. Fast, flowy, and loads of fun. I've owned one a bit more than 2.5 years, and except for needing a light shop job, it shows no sign of getting old. Some call it shallow, but that's ok for this game. Portal is very difficult to get to. It just has a zippy bitchy quality to it. A nice risk reward for some shots, and just plain fun. The spinning disc is so simple, and irritating at the same time. It can be a bit of a quick hitter with brutal games, but every once in a while it will reward you. Sea of Simulation (SOS) is a nice pre-wizard mode to let you get the modes you missed (Lightcycle). Good dots, good sound. Requires a subwoofer to get the full sound. Great game.
STTNG is greatness from the 90s. The one knock I will give it is that it can be a bit of a pig to maintain. The cannons can be a pain. But fully working and lit properly (one of the few games I like LEDs in), it is a great shooter. I don't like the video mode because like most it is just a pattern. The modes are fun, the Picard Manuver is a winner if you hit it repeatedly, and its fun! I always like the Rescue mode for some reason. And the jackpot calls are just right. It doesn't quite feel like other games of the era. It always feels big. But it is always a good play. Don't cheat with lane extensions or anything (very common on the game).
So overall I'm going to say if you want a quest, LOTR. If you want to get slapped in the face by a pretty girl, Tron. And if you want one of the best from a great era of pinball, STTNG. Every one of them is a winner.