How many series are there out there where the original actually is the best game in the series?
There are a few out there. You mentioned
Contra and it was the first one I thought of. While
Super C,
Contra III, and
Hard Corps are all fantastic games in their own right--adding new dimensions to the game play to keep things fresh--they just don't have that put-in-and-play simplicity of the original. It's just a fantastic way to kill 45 minutes.
I feel the same way about the original
Castlevania. Speaking of, if we count the "Metroidvania" games as their own thing, then SOTN was heads and shoulders above all that followed.
Another original I think has that put-in-and-play charm is the fist
Dragon Quest. Certainly each subsequent game added new ideas and the stories became more involved and all that, but there's just something about the sheer simplicity of it.
Now that I think of it, "the first is best" seems more common with RPGs. I've already noted up-thread that I've always preferred
Chrono Cross to the original, but it's hard to deny it's probably one of the ten (if not five) best videogames of all time. It's a bona-fide masterpiece.
I think the first
Xenosaga game was the best. The latter two, while still maintaining a high-level of story telling and music, they were underwhelming experiences overall. (The gameplay was just really terrible in both cases.) And if we can include
Xenogears, then it's definitely the best.
I think
Tales of Phantasia is still the best Tales games. The whole franchise has become so derivative and generic--especially with the current generation.
As much as
Suikoden II &
V are hailed, I still like the original the best. It, too, has that whole simplicity and charm thing working for it.
The
Final Fantasy Tactics Advance games are pretty terrible. Some don't even considered them (or want to consider them) as part of the same series as the original.
Sqeenix's other TRPG series
Front Mission also peaked with the first one. Though I hear the last one (actual game, not that silly spin-off) was quite good. But by the time that came out, I'd stopped importing, so I've never played it.
KOTOR is the definite winner there. I prefer the second's story, but overall the game is a mess--even with the "fixes." And despite the fact I spend most of my gaming time playing SWTOR these days, it's just not in the same league.
Neverwinter is another Bioware RPG game that spawned and awful sequel. And for what it's worth, I actually like the first Baldur's best (including IWD). I know I'm in the huge minority though.