Snapper first, Ariens second, Honda third, Toro fourth. Older Toros were better, I'm not a fan of the new ones. I'd tend to avoid plain Recyclers--go for the Super. Deeres are decent mowers but complex and overpriced. Lawn Boy has been ruined as a brand in the last two years.
2-strokes are now only available nationally on commercial mowers. Outlawed for civvie sales.
Skip the MTD/AYP/Murray machines (this includes Sears, Noma, Yard Machines, Cub Cadet, Husqvarna, Troy-Bilt, and more) unless you're getting a very simple one without drive, BBC, and other options.
If you really, really want my detailed opinions of mowers I'll see about digging up my past OE (outdoor equipment) board posts.
What brand is yours that you need parts for?
I'd tend to go used, but that's just me. I repair them as a hobby. You can get great deals.