Several sets are missing:
The 1928 PCL issue is 32 of the worst pains in the neck you will ever chase. I'd put them right up there in the top 3.
I'd also put completing the PC backs up there towards the top because of the short prints:
Also, the mixed baseball-boxing PC back 4 on 1 series: as far as I know no one has a complete set. There are four known cards but a fifth is very likely (details to be shared once I finish writing them up for my blog):
Also, the PC back 4 on 1s w/4 ballplayers. Very few of those around and an uncatalogued one (Miller, Collins, Kelly, Klein) just sold in the last Legendary auction:
