I'll second the XRGB3, it does S-Video really well. The budget (Composite+SVideo) transcoder solutions are all really bad. I've bought at least 6 of them hoping the smaller shell meant different motherboard innards that didn't suck - all are equally shitty from my experience.
For less budget I've usually had Google steer me toward Altona products. I own two of their products, and have called their tech guys to ask questions before I ordered another. The solid answer is these are strictly scalers. You will never get 1:1 or 1:2 etc scaling. They all run the same SOC scaler and it is not capable of a clean 1:1 picture. Very blurry and slightly green tinted in the darker color range. Also, lag.
I have been able to tune a PC video capture card (with S-video input) to feed raw YUY2 data to some optimized filters and get a good full-screen image output to VGA without too much lag and a perfectly tuned image (still interlaced, to reduce lag), but this took a lot of trial and error with a hacked driver back in the day and I was using the composite out (multi-output video cable from the consoles) to play the game while capturing the raw video due to the slightly still-present lag & interlacing. With a quick deinterlacing filter running, the game was beautiful and playable on a crap computer that I had in 2009. Probably a decent used usb3 capture card nowadays would easily beat it...?
I have seen some project/bread-board solutions. These are off the shelf parts that turn S-Video Chroma-Luma to RGB 15Khz, ready to be output through SCART or BNC. They were solutions for arcade monitor enthusiasts. If they were simple, I'd have made one up for compatability to my various hardware, but I didn't really need one due to RGB modding I've done to all of my retro consoles (oldschool NES PC10 and SNES done by Jinn); It's easier to pull RGB off of most consoles motherboards than it is to make the S-Video transcoding board you'd have to build. Google will steer you right if you want to make your own solution with ~$20 parts from Digikey or Mouser. Post a topic in the hacks forum with the parts list and I'll play along with you. I do want one of these transcoding boards so we can build it together. Mouser's parts take 3-6 days to arrive to me if they're in stock.
I did make an S-Video to Chroma & Luma RCA cable so my monochrome displays didn't have colour fringing. It's super effective. The 9" green monochrome monitor never looked so crisp and retro with my Saturn and PS1 games.