Not much to say that hasn't already been said. A.k.a Im too lazy to write a full POTW post, because Jellicent is so boring (no offense Jellicent, it's just you're way too common and I use you too much). Anyway, Overview:
Say hello to the gen V's premier wall. One of the most common walls out there thanks to its versatility and typing. Defensively, Water is great typing, and Ghost is even better, having immunity to two, count it, two, types. Not only that, but it has immense Hp and really great Special Defense, along with a very usable Defense stat. Great move-pool all around, with everything from Recover to Will-o-Wisp, from Taunt to Scald. It's great! Not only that, but it goes with my conspiracy that Gamefreak is running out of ideas, being a very similar pokemon to Tentacruel stat-wise and inspiration-wise. Speaking of looks, the only really bad thing about Jellicent is, well, its looks. It is strange looking. The only thing keeping it afloat (pun intended) is that amazing mustache and cute crown it has. In a nutshell, it's a balloon with wings.
But I digress. Besides being amazing stat-wise, typing-wise, and move-pool-wise, it has great abilities with Cursed Body and Water Absorb. One gives you a third immunity, while the other potentially wrecks choiced sweepers.
Jellicent is that pokemon that Gamefreak just loved when they made it. Either that, or they wanted to give competitive battling a present. Jellicent has literally everything it needs to be successful. However, that doesn't mean it's perfect. Toxic is its worst nightmare, and 999 times out of 1000 Ferrothorn will successfully wall it and stall it out. It can really hurt other teams (and take pain from them), but this big fatty can't take its own medicine.