There are two sequels I prefer over the original film, and they're both by James Cameron. The first is 'Aliens', because, first of all, this time there's more than one alien. I also find the sequel to be much more action-packed - the first 'Alien' was more like 'Ten Little Indians' in space. This time, there's more potential victims, and more aliens lurking round the next corner to grab you. Plus, Sigourney Weaver's Ripley is just a badass. Her fight scene against the alien queen is classic.
The second is 'Terminator 2', which continues a decade after the first movie, but this one goes to 11 compared to the 1984 original. Arnie's a good guy, Sarah Connor is a killing machine in her own right, and the T-1000 is unstoppable. Add Guns 'N Roses to the mix, and the first movie looks like a Merchant Ivory production.