It all comes down to preference on what type of character you like to play.
Rumor has it that Summoner's are one of the best/easiest classes starting, but are harder to play in later levels.
Bagi's i'm not sure about, but at later levels they have some pretty good AoE's.
Knight's with a sword/shield build are awesome tanks
Hunter's, whether you go dagger or bow (dagger seems to be more powerful) are both pretty good at higher levels.
I'm not sure about Mage's at all, they seem good but I hear their build is fairly complicated.
As for a Segnale, I started and have only played one. They level slow as hell up until level 59 (when they get Curse Field, arguably one of the best AoE's in the game) then they start to royally kick ass. As a Segnale, you would save a fortune on potions, I rarely ever use a health potion and I get by on cheaper mana potions.
However, even at higher levels I would think that Segnales are one of the hardest classes to play since you don't just have to watch your life, you have to watch your parties life as well. Not to mention people expect us to constantly be attacking too.
Basically, what people expect of a Segnale-
Healing for the entire party.
Buffs for the entire party.
Attacking constantly (mainly once you have Curse Field).
Keeping a constant eye on the chat log.
To revive a party member if they die.
As to the main downside of a Segnale, alot of other people look at us as a weak target for easy pking, so what I do is keep a list of everyone who has pk'd me in notepad on my middle comp and those people don't get healing services, party services with me or resurrection when they die. It works well, so far several of the people on my list have begged me for help and i've denied them/laughed at them.
So.. let's split it into good/bad for a Seg.
GOOD-
Save $ on pots.
Heal spells (including Fast Healing, which heals per second for 90 seconds as well as a fairly large solid number on intial casting and OWNS).
Resurrection of other players.
Generally easy to get into a party.
Party buffs or single target buffs for guard rate/defense/phys & magic dmg.
Curse Field!
Pretty easy leveling once you're 59+.
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BAD-
Easy pk target.
SLOW leveling up until 59.
People expect alot of you, easily twice as much as they expect of any other class.
Alot of dumbasses don't use potions when a Segnale is in their party, they think heals and res don't have cooldowns.
Hard time soloing things between about lvl 25-59, it is doable but other classes do it easier.
People charge a LOT more for legendary Segnale gear than they do for legendary gear for other classes, in general.
Ultimately it's up to you... I would go with Azure Knight, Bagi or Segnale though.