Soul Reaping certainly is powerful, but don't forget Warriors have Adrenaline (meaning they can take advantage of two different resource systems) and Rangers have Expertise which gives a lot of advantages as well.
None of the classes are super hard, even Mesmer can be pretty straightforward nowadays, although this doesn't play to their strengths.
The one downside to starting in Prophecies, and only having Prophecies this late in the game's life is that you're going to find it difficult to party with live human players. This presents a myriad of problems. One, you won't have Heroes. Two, playing a support or healing class (i.e. Monk, Necromancer and Ranger) may not be as fulfilling or fun. The Monk you'll mostly be redbarring Henchmen, the Necromancer will be hanging back and summoning minions (the best build to use with Henchies generally) and the Ranger is best at interruption and single target conditions.
A Warrior can run into a group and has the armor to withstand a lot of damage, allowing hench monks to keep you up while you do pretty good DPS, and is even more rounded if you slap Sentinel insignias on your armor. An Elementalist can of course nuke, and is still effective even in Hard Mode, but probably not as versatile as a Warrior. Mesmer can offer some fun, but if you're looking for relaxed, casual gameplay, an interrupter/controller is probably not the best choice.
Overall, I'd say Warrior is your best bet, as they are easy to learn but difficult to master, giving you play longevity. As for secondary, you might consider Ranger for a pet (always nice to have a companion), Necromancer for Plague Touch or Monk for Mending Touch (Blind, Cripple and Weakness are the bane of Warriors). Ranger can also acquire Melandru's Resilience later on during the Dragon's Lair mission which can help a lot in the Ring of Fire missions.