I was about to point out that vagrents can't really be considered rarities as, in the normal course of things, the wouldn't occur here anyway. To me a rarity is something that you can (sort of) predict. By that I mean that you are in the right habitat, at the right time of year, in the right location, but due to extremely small population size, nomadic habits, or extremely shy and retiring behaviour, the chances of spotting species X is very low.
However that said, to me, and this is very much a personal opinion, species like the Pel's (seen, Shakawe, Jan 2008), Narina Trogan (not seen), and Blue Swallows (not seen) are not 'real' rarities. Their numbers are extremely low but the nesting sites are closely monitored and providing you go to the right place, chances of seeing them are very good. The same applies to Swynnerton's Robin (seen, Dec 2007, Bvumba), althought it was a thrill to see it, some of the joy was lost because we walked to a spot, spished and behold! it appeared.
On the other hand, seeing a Long-Legged Buzzard (KTP Feb 2008, unconfirmed by rarities committee, but that's what I ticked it as) on a random game drive was an amazing and very exciting sighting! And so was the group of Wattled Cranes spotted over theeeeeere on the flood plains in north-eastern Namibia.
I personally feel a bit like I have cheated myself if I can walk up to the nest, or known roost of a 'rarity' and tick it (like the Pels). However, ask me again when I've reached the 600 mark and I am battling to see new ticks!! I will probably have a very different opinion!!
Off the top of my head, my most memorable and 'rare' sighting are:
Brubru pair on nest
Ground Hornbill (parents & 1 juv)
And there are probably more but I can think of them at the moment!