I've never played a BoF game. How are they?
Varies.
1 - A very conventional SNES RPG with a neat (OP) dragon transformation gimmick that otherwise trends below average in terms of design. You do have a wide array of playable characters to choose from but as was common for the time one strategy tended to dominate. Dungeon design is less than inspired. Story is functional.
2 - Wildly overrated. Dragon powers, the main draw, are less fun than in the first game, and the game suffers from an obscenely high encounter rate. Story is the big reason this one gets praised but it wasn't anything special even 20 years ago, well after release, let alone now. Original translation is also butt, but there is a patch that fixes that at least. For my money, this is the worst game in the series because of how poorly it plays. At least 1 favors blandness.
3 - This is where we start getting to the good stuff. High quality sprites, very creative dragon system (mix and match genes for combos and specific forms!), the introduction of character customization (for level up in addition to a base set of skills). The game just being a fun, mostly light-hearted adventure (with only the occasional dark undercurrent) is a big draw for most people who cite this as their favorite in the series. Also that dragon system. It's so good, man.
4 - Another solid title. This game's aesthetic is much more washed out than the previous entry's colorful appearance, but it fits the more somber mood of the story. Most of the game systems from 3 return with varying levels of improvement, except for the dragon gene system, which hurts especially hard because of how late into the game you have to progress before you can really toy with them. Battle system improvements shouldn't be understated though and largely(more than?) make up the difference. Also a dual protagonist system that most people are on-board with because the secondary protag is a cool dude.
5 - Very underrated. It branches from the previous games' traditional SNES/PS1 JRPG globe trotting paradigms to be a dungeon crawler that also heavily punishes you for using your intentionally OP dragon powers. It's also a short game (10-15 hours) that has systems in-place that incentivizes players to start a NG+ in the middle of their first NG due to difficulty and rewards specific to NG+ (and NG++) runs. Difficult to explain more without going into a huge spiel. This game gets a bad rap though because of how incredibly different is from the last games - These had always been second-rung genre titles and its relatively niche fanbase didn't appreciate the changes from convention. It's really good though.
6 - *Doesn't exist*. Feed the gacha.