They all have their individual advantages and disadvantages:
Tekken 1: The Good -- it was pretty original (lets not start a "Tekken copied Virtua Fighter" argument though

). The combo system was a great start, and the characters were easy to fall in love with.
The Bad -- some combos took off way too much, even 100%! The characters were not balanced very well either.
Tekken 2: The Good -- by far the best music and best environments. King\'s stage was beautiful and had the music to match. The characters are much better balanced than Tekken 1 also. The new characters were memorable, the storyline was at its prime, and it was released at the time when arcades were still very popular. (I MISS THOSE DAYS!)
The Bad -- There were still quite a few clone characters.
Tekken 3: The Good -- This is probably the most balanced Tekken game (iirc). There were still character tiers, but they weren\'t as spread out as in other Tekkens. The clone characters became their own person for the most part with their own move lists.
The Bad -- Eddy.
TTT: The Good -- Great graphics. A whole lot of characters. The return of old favorites (from T2

).
The Bad -- character balance is horrible. PS2 version\'s music sucks compared to the arcade counterpart and to previous Tekkens IMO.
Tekken 4: The Good -- The death of some clones (Armor King, Kunimitsu, etc). The characters\' move lists were added to enormously again. Steve Fox (very original).
The Bad -- character balance is once again messed up slightly, with Paul and Jin being the biggest culprits of this problem. The complete change in gameplay style (no more backdash?!) Over priority regular punches. Blah blah blah. Too many problems to go on listing.
I wish I could spend more time with Tekken 4, but as of right now, I\'m thinking of just waiting for a souped up Tekken 5.

Tekken 4 was only in development for around a year, and it unfortunately shows.
As a fighting game, I think that Tekken 3 was the best. It had the most polished fighting engine and most refined gameplay. For overall presentation though, Tekken 2 wins hands down.