For slides, anything with a slide feeder!
I'm gradually scanning my negatives and slides. 200 rolls have taken me about 3 years so far. Don't do it my way :-)
I've got an old Canon 4000US, which is OK. I had a Minolta before that which had terrible trouble focussing. Today, I'd probably get one of the Nikons.
I prefer a dedicated film scanner over a flatbed for my negs because the Canon, for example, has an infrared channel that detects dust, and that channel info can be used by scanning software to fix the image automagically.
The latest versions of Hamrick's Vuescan can save files in DNG format, so now I'm just scanning as DNG and JPG (the latter for reference) and will fix the DNGs later as and when I ever want to look at an image.
For flatbeds, assuming you're scanning in bulk, you need something with a straight-through feeder. I've recently started using one of the Fuji double-sided scanners. The image quality is adequate but the speed is unbeatable.
If I were starting over, I'd find someone else to do it and simply pay them, to be honest.