Snivy: Decent, assuming a physical moveset and at least neutral nature. Has enough bulk to set up without too much trouble, and Coil certainly improves Aqua Tail, which is pretty much a "must-have". Never really loses any steam in-game; has trouble with Roxie and Skyla, but none of the other gym-leaders are particularly problematic.
Espeon: Amazing in-game. The combination of a high Sp. Atk and Speed, and availability early on (pre-third gym) , make it an excellent sweeper. Physically frail, but has a significant amount of special bulk. Evolves by happiness, but has an above-average amount of happiness to begin with. My Eevee evolved by the defeat of Burgh, and Espeon proved invaluable from that point on. Using move tutors is recommended; Signal Beam helps with its rather limited movepool.
High Tier, possibly Top with Signal Beam.
N's Zorua: Another Pokemon that has incredible power in game. Comes at a relatively low level, but as it has a different ID, levels up quickly and evolves within five levels of obtaining. Has a relatively large movepool, if a bit lackluster without TMs. Frail, but speed and sheer damaging potential makes up for its lack of defensive might. Can easily deal with half the E4; Shauntal and Caitlin fall with no issue what so ever.
Venipede: Not altogether too useful; obtained early, but lackluster through entire game from that point on. Has a remote amount of use against Burgh, I suppose?
Azurill: Genuinely quite useful, in my eyes. Huge Power is needed, of course, and Azurill itself is nearly useless. Marill and Azumarill, however, are of major use. Azumarill has a decent movepool between Aqua Tail and Return/Double Edge; both attacks are gained very early. Superpower is of some, but not much, use. Easily KOs most of Team Plasma's, including Ghetsis's, Pokemon. Is rather slow, but makes up for this with sheer strength.