I think glaive (or other polearms) is the best pact weapon in the long run, due to 2x 1d10 + str + cha mainhand attacks and a 1d4+cha offhand attack. It also gives you an extra hit to trigger Hex damage. At level 5, it should be roughly equal to the greatsword, or about 3 points ahead with hex. You also get extra utility with the attack of opportunity on approach etc.
That being said, you never get more than 2 attacks (3 with offhand bonus), so just cantrip blasting is better in the long run. Melee pact maxes out around 52 damage (using +1 weapon, str/cha both at 5 and average damage rolls... 5.5 / 2.5 main / off, 3.5 for 3x hex), cantrip is 4x 1d10+5+1d6 = 56. You have to wait for level 17 for the cantrip blasting to top out, but it's easier on the attributes.
Both are viable, but there's not really a lot of payoff going for a pact weapon, aside from it being cooler.