Suikoden 5 is very much a great game. I'd say it's the best after 2. If you have the opportunity to play it, definitely do.
The only bad game in the franchise is 4. S3 is divisive because it changed the combat system (though it's still good), and the story has three protagonists whose chapters can be played in any order for 2/3rds of the game. Mind you, it's still a good game. It's actually the game that introduced me to the franchise after I read Gamespot's review of it. I liked it enough to play through it twice!
So yeah, in your case, I'd say play any of the first five games except for 4. You can't go wrong. If you just want to play one of them, then go for 2, and 5 after.
