Can be any class or class kit that can make a legal dual class into another class. Whether for reasons that involve the abilities of the first class or the lack of good compatibility with the class(es) that it can be dual classed into.
I would probably have to say the Wizard Slayer myself. The advantages themselves aren't really that great to begin with as other classes do what they do but better and without the restrictions they have. Plus, if dual classing then your magic resistance that you will have is going to usually be a non factor at 15% or less, it's rarely going to be able to save you in a battle and in BG1 levels it won't offer much protection even against high damage spells. Possibly the worst part is you won't be able to make much use of magical equipment, particularly magic items meant to be used by your 2nd class. So if you wanted to use some incredibly useful items then it's a bummer, or give yourself some equipment to raise resistance to something coupled with your innate Wizard Slayer magic resistance for a little extra.
Something around 10-15% magic resistance by itself usually won't help much, but having something that gives you 50% resistance to something like fire damage means that 65% is a good thing to have over just 50%. But you probably won't be able to get much of that without specific weapons or armor, and are very reliant on your base stats. Speaking of stats, you'll have to have the prime stat of the class you want to dual class into at least a 17, drawing points away from the physical stats you'll be very reliant on without the benefit of so many magical items.
The Kensai and the Berserker kits simply put, make better dual classing options than the Wizard Slayer cousin. And some of the Paladin kits would be better dealing with spellcasters than would a Wizard Slayer in all likelihood. The Cavalier with their passive resistances and immunities, the Inquisitor with their powerful debuffs, the Blackguard with their Poison Weapon ability being excellent for disrupting spellcasting. Of course you can't dual class a Paladin, but they make being even a dual classed Wizard Slayer redundant cus you can get a kitted Paladin NPC in your party, or simply be a Paladin instead of a Wizard Slayer dual class since there are better dual class options out there than using a Wizard Slayer for one.
Well anyway that's just my take on it. What does everyone here think the worst class to start as for a dual class is?