Suikoden2 is not only my favorite, but it was the first Suikoden I played as well as the first RPG I ever played. I've played quite a few RPGs looking for one that's as good and I've never found anything close, it remains my favorite game for nearly 10 years now.
Suikoden1 is my second favorite, though I have to get pretty board to go and play it again, it's just hard cause it's so primative in comparison to even the second game.
Out of 3-5 I don't like them as much for different reasons, though there are things I like about them.
Liked: I liked that they added skills, it was a nice addition and a good way to add more depth to making your characters more powerful and allowing for more versatility which is the main thing I like about Suikodens, they have excellent Replay value. I also liked how they moved on the battlefield, though I prefer the classic Random Battle format it wasn't a bad change.
Disliked: I didn't care much for the whole "Pairs" system they used, it limited your control over the party a bit, though this can also be seen as a challenge to some so it's not a entirely a bad thing. I also didn't like how they seemed to have slipped with character design in 3. In my opinion a lot of the characters just weren't as interesting as the ones in pervious games, though it had some good ones. Also they seemed to have added a lot more children, the older games had maybe 2 or 3 kids, but that was it. Suikoden3 seemed to have a lot more youngsters running around.
All in all 3 is probably my favorite Gamplay-wise out of 3-5
Liked: The first one to include voices
a nice additon, though not unique to the Suikoden series by any means. I also REALLY liked the training room. it was great both for testing your party and having a challenge as well as maxing out your characters. I also was THRILLED that they randomly included Ted and allowed you to use the Soul Eater. I also liked that they let you construct equiment, I'd like ot see them add that to a future game in the series.
Disliked: I think my biggest problem with 4 was that they cut the party size down to 4 from 6. In a game where one of the primary details has always been such a variety of characters I don't really feel it did justice to the series to limit you to 4. I also didn't care for the boat or the island setting, but that's a matter of personal preferance for me and not a flaw in the game. Also they REALLY were lazy when it came to graphics for the runes, I mean even some of the spells in Suikoden2 were more impressive than most of the ones in 4.
This is my least favorite out of the main series.
Liked: I liked how in this one they went back to more like the way 1 and 2 were. Also it had the best character design (in my opinion) since 2. And they brought back Georg (my favorite character)
Disliked: I personally felt that this game seemed to include WAY too many parts that were just running around and talking and not really enough Dungeon/Boss parts, I mean some story parts are needed of course, but it just felt like the cut scenes in this one went on forever.
Out of 3-5 I liked this one for the characters it had and that it's most like 1 and 2 out of the newer games.