Again, I say HP Priest is fun, simply for the sheer juggling of using claw and Heal without MG. But it's not for most ppl, so second, I'd say Warriors. It seems lame, but once you get final attack, and all the bells& whistles it's extremely cool. Plus, the classes unlike Bowmen and Magicians, couldn't be more different. You have an elemental sword user, a combo user, and a mass effect/support user, all with very different ways of using the same stuff. Plus, the type is very forgiving, you can go Dexless and still hit, you can go up to 2 Dex until about level 30 and still deal decent damage. Go with Warrior.
Personally, I've never really had much skill with thieves, so I wouldn't recommend this. Bowman has too much cost of rebuying since you can't simply buy rare arrows and recharge them, unlike the other ammo classes, you have to keep buying ammo batches. Pirate has basically the same makeup as thieves, but for str and dex. In other words, it should be like a warrior, but suckier defense usually gets in the way. I ended up using alot of pots on this one (although not quite as much as later with the page due to Slash Blast). You could try magician, they have sucky defense like the pirate but at least they have ranged attack which restores with time instead of with money spent. Or you could try the newest thing, Aran, which if you're GMS comes out tomorrow.
Btw, if you're bored with any class, be sure to sell any rare items for the max they're worth. I decided my Page was still too expensive to train outside PQ (the Final Attack being mastered would save the money, but it was still a few levels off), and I was having trouble with CPQ2. So I went to FM and sold my fairly godly hammer (+15 STR) for a bit over 11mil, and sold the Dark Crystal I had saved up for another mil, and then sold a bunch of the other items, making over 500k. I bought a cheap Polearm, and put any really good armors I'd been saving into storage in hopes they are usable by the Aran. The only regret is that I couldn't get more use out of the class, because as fun as it was, I was still wasting pots like crazy. Still, I recouped my losses from expanding my guild.