The Witch King has actually won plenty of battles and these are for the most part described in the Appendices in RotK; for example:
In 1974 the power of Angmar arose again, and the Witch-king came down upon Arthedain before winter was ended. He captured Fornost, and drove most of the remaining Dunedain over the Lune; among them were the sons of the king.
This was the guy who co-ordinated the destruction of the North Kingdom and ended the line of Kings in Gondor, after all. That makes him a force to be reckoned with.
The instances you mention are quite specific exceptions. Let's look at them and see why.
First of all, after the attack on Weathertop, as far as the Witch King was concerned he had already won. He'd stabbed Frodo, the fragment of the blade was working it's way, and it was only a matter of time before Frodo was overcome. He didn't particularly need to fight, so it's a simple enough strategy to back off and wait. Of course, being unaware of the resilience of Hobbits he calculated wrongly there, but that doesn't make him "weak".
Regarding Glorfindel, here we're talking about a Lord of the Noldor from the Blessed Realm who had slain a Balrog in the First Age. As Glorfindel himself says, he is one of the "few even in Rivendell that can ride openly against the Nine" - that doesn't make the Witch King "weak", it makes Glorfindel incredibly strong.
Gandalf as a Maia should be obvious too; particularly in his "Gandalf the White" incarnation. The Witch King may be a powerful servant of the Dark Lord, but Gandalf is a spirit who assisted in the creation of the world and a being of supreme power in Middle-earth:
'Dangerous!' cried Gandalf. 'And so am I, very dangerous: more dangerous than anything you will ever meet, unless you are brought alive before the seat of the Dark Lord.'
Finally there's Merry and Eowyn (I'm skipping over Arwen as her involvement here is just movie foolishness), and their part in this is fulfilment of a prophecy; just as Frodo was meant to find the Ring, so were Merry and Eowyn meant to destroy the Witch King (Merry possessing an enchanted blade with powers for just this purpose didn't hurt either). The whole topic of destiny in Tolkien (or Doom as he would have preferred to say) is a huge one that could take an entire book to explain, so you'll just have to accept that it's there, that it's important, and that when it happens notions such as relative degrees of power don't matter much.
It's not that the Witch King is weak, it's that you're seriously underestimating those who came up against him in the main narrative.
