I'm actually a big fan of the original Tron. I found it imaginative with engaging performances. Genuinely can't understand the hate it gets. I think it still stands up today as a piece of out-there sci-fi. Also, cutting edge for its time.
I think everyone probably has a soft spot for Split Second with Rutger Hauer. Cheesy and derivative in the extreme, but I always found it impossible to dislike.
I haven't seen it for a long while, but I always remember being quite fond of Slipstream - the one with Mark Hamill. I found Bill Paxton's character's fascination with balloons rather sweet, and I was always a fan of the late, great and supremely underrated Bob Peck (the gamekeeper in the original Jurassic Park), who played an android from memory.
The ultimate B movie, of course, has to be Tremors. I really must try and watch that again.
Looking back through the list of films other people have posted, I'm mindful that perhaps none of these qualify as legitimate, old-style B-movies though.