More or less what Pertho said. I’ll elaborate on the individual games, but there are a few things that should be mentioned for T7 specifically: a lot of the things most people thought were frustrating when learning older Tekken games were simplified in T7. The most notable parts here are that older games required you to hold the back input in a backdash if you wanted to block during backdash, while T7 also lets you block during backdashes while holding neutral, which makes it less likely to be clipped while backdashing. Throw breaks were also a lot harder before (shorter window, and everyone had 1, 2 and 1+2 breaks), though throw breaking is still really hard to get good at and some characters still have 3 breaks. And, finally, okizeme got changed really drastically. In older games you could easily get re-floated if you got up wrong after a knockdown, and while that’s occasionally the case in T7, holding backwards when knocked down lets you get up safely-ish 90% of the time, with the caveat that you probably have to eat a mixup.
So. The other games.
T3 and TTT1 basically invented the basics of Tekken 3D movement and stuff like that. I like both of them, though it has to be said T3 has a bunch of really dumb mechanics (a bunch of moves gave free hits if you blocked them) and Ogre was incredibly dumb, while TTT1 was somehow even worse in terms of balance. Still, both games hold up even today. One of my local bars has a T3 cabinet, so I’ve spent some evenings drinking beer and playing on that with buddies.
T4 had a lot of cool ideas, with varying results, and really great presentation even today. However, it has the weakest overall movement in the entire series (short backdashes and sidesteps), which really throws a whole wrench in the whole thing when movement is what sets Tekken apart from other 3D fighters. Jin (and to a lesser extent Steve) has also been mentioned already. It’s worth taking a look at, but it’s definitely got some big issues.
T5 / T5DR brought back movement with a bang, and is definitely my favorites in the series. You’re gonna feel like you’re moving on rollerskates compared to T7. Both games are great, though I’d say T5DR is still superior in every way. More characters and better balance in all ways; a bunch of the cast in T5 were hilariously busted and fairly braindead: case in point, Heihachi had a 10f punish that did 45% health without the wall, was safe on block so you could kinda just do it, and he wasn’t even top 5. T5DR had most of the cast be viable and powerful, and even though the Mishimas were pretty stupid in the right hands, they weren’t everywhere simply because of the execution barrier.
T6 nerfed movement again and made the game more poke-oriented. Unfortunately they screwed up on balancing the game, and the divide between the good and the bad characters was very noticable. Still a good game, but I think T5 was better.
TTT2 struggled with bloat and being hilariously complex. It was a combo nerd’s dream, to be fair, because the system was incredibly open ended, and movement was buffed compared to T6 again. However, there was just… too much stuff. This was the first Tekken game I tried to learn, and it turned me off until T7 dragged me in.