You have a few options, but most of your optimal choices aren't actually swordmage paths.
Malec-Keth Janissary (MotP) is really the only swordmage path worth looking at; all the other good ones are for shielding/assault builds or have racial restrictions. It's got relatively good powers (errata gave the encounter power a boost so be sure to check that), and the level 16 feature lets you add any of several different elemental types to your attacks, which opens up some decent feat combos (frostcheese, for example) and is an easy way to trigger vulnerabilities.
Academy Master (D374) is open to all arcane characters if I recall correctly. It focuses around boosting your at-will powers; the encounter & daily powers can be used to add some oomph to an at-will. Pick up White Lotus Master Riposte and you'll be in pretty good shape. A reliable, if not exciting, choice. Per the rules, you can only use 1 White Lotus chain with an at-will at a time. Thus, Riposte & Master Riposte are all you really need; other White Lotus feats will just give you more options. If for whatever reason your DM is unaware of this rule or doesn't enforce it, then this path becomes an excellent option, and you should load up on White Lotus feats ASAP.
If you're willing to multiclass wizard, Blood Mage (PHB) is a classic, and still a decent choice even after repeated nerfs (if you can use the original version it borders on overpowered, though sword mages aren't the right base for abusing it).
Another wizard path worth looking at, Unseen Mage (AP), is all about turning invisible. While that's not amazingly useful for a wizard, it can be highly entertaining for a defender. While shielding and assault swordmages generally have better options, this one certainly isn't a bad pick for ensnaring swordmages.
There are some other options that are not such good choices.
Wandering Swordmage (FRPG) is a great path open to all swordmages, but it rewards (and practically requires) a heavy investment in wisdom. While that works great for ensnaring swordmages since they aren't nearly as strongly tied to their secondary stat as shielding and assault swordmages, it's something you really need to plan for from the beginning. As such, while it could be a great choice for other people who find this answer while creating a character, I can't recommend it for you. Important caveat: Wandering Swordmage was published before the ensaring build was (in the same book as the swordmage class, in fact), so it doesn't reference it; the class is clearly not intended to be limited to just one aegis type, though. If your DM isn't a stickler for RAW it shouldn't be too hard to convince them to let its features work with ensnaring aegis; if they won't then stay far, far away from this path.
Ghost Blade isn't a bad choice; you'll never say, "Man, this path is terrible! What was I thinking?!" It's not a power house, though. The features mainly just give you combat advantage and increased teleport range, while the powers are all lackluster (the daily is good for clearing minions, but that's not your job, plus it requires you to burn some other resource to be phasing or insubstantial). The encounter power in particular makes it clear this path was designed for assault swordmages, and they have better options anyway.
Orien Swiftblade is also a so-so choice. The powers aren't bad, the features are underwhelming but not complete junk; the real problem with the path is that it requires you to take the Mark of Passage feat. Don't get me wrong, Mark of Passage is a nice feat, especially for someone who teleports a lot. Taking Mark of Passage is bad because it means you can't take Mark of Warding (you can only have one dragonmark feat), and Mark of Warding is downright amazing for pretty much any defender.