Forget Frightful and Sickening Blast. They don't scale and enemies are far too likely to make the save to negate them. Go with Entropic Warding (it's awesome, but you will need to decide what the duration is with the DM. The two spells it mimics have different durations).
Hammer Blast (Complete Mage) has its applications, but generally Baleful Utterance is better. If you want to be the face of the party, I must reccommend Beguiling Influence and Call of the Beast. These two can get you tons of information that would otherwise be far more difficult.
So, for my seven invocations, I would choose:
Baleful Utterance
Spiderwalk (godlike manuverability at low levels, remains handy at higher even though it's generally replaced in functionality by Fell Flight later)
See the Unseen (better than Devil's Sight, no question).
Beguiling Influence (since you want to be the party face)
Call of the Beast (ditto, the more sources you can get, the better)
Entropic Warding
Campaign dependant:
By this last one, I mean it makes a difference on the campaign. If you'll be around a lot of water, Swimming the Styx is worth it. If you won't pass low levels, then Summon Swarm is excellent (remember, you have no way of swapping invocations!) If you do a lot of spying, then All-Seeing Eyes would be a great choice. If you plan on doing a lot of tracking or being amubshed, I cannot reccommend Sperent's Tongue enough for Scent (also not a bad choice if you find yourself having poison problems) If you think enough fights will start at long range, then Eldritch Spear is worthwhile.
If you honestly have no idea, then I would reccommend either Eldritch Spear or Dark One's Own Luck. Extra range or saving throw bonuses never go amiss.