For each line, that's a tough one, especialy when ousting the exclusives.
I live in a retail heavy area an can find most things mainstream released but off the top of my head some problem bots have been
Titanium Cheetor, G1 Ultra Magnus, WW Ultra Magnus, The Fallen and SunStorm.
I only saw The Fallen and WW Magnus once, sadly I passed on WW Magnus(I bought 2 of The Fallen online so no biggie). Cheetor I saw twice but sadly only bought once(I won't open a Titanium till I have a dup). SunStorm I've never seen in retail.
Then Theres always the specific Combiners like LeoBreaker/Nemisis Breaker and WingSaber that you know you'll only see a handfull of times. Nemisis I got online, WingSaber I knew score right away.
If you're vigilant, retail bots will not be elusive, hell somehow I ended up with 3 08 BumbleBees with out even trying and I could have upped that number greatly, but I'm a collector not a hoarder.
But the key is, Remain vigilant!

