If it's cheap buy it, if it's crap and you never watch it again fair enough but at least you get to see him in something you haven't seen yet.
Sherlock, hands down. It's just such a good series for other reasons as well.
Film wise it's harder because like everything it depends on the film subject if it's really enjoyable or not (I've only watched Black Mass the once for example). I like him in Tinker Tailor, but the ones I've probably watched the most would be the Marvel/Hobbit ones
Theatre: Frankenstein.
There's actually quite a few things I haven't seen yet, my Cumber to be watched pile is quite big.
Which is your best Henry?