A Herpstat Pro is a bit overkill for a single rack application, though the basic Herpstat is a great unit. I prefer it over the Helix, though that is probably because I used the Herpstat first. I have one Helix, and it has never given me a bit of problem (except the time it got unplugged, but I can't really blame that on the Tstat, lol).
I've heard of a few issues with Ranco's over the years, but have never used them - a lot of people do though, and with great success.
I like the Johnson Controls A-419, personally. I've been using them for years, and the only problem I ever had was a power strip crapping out - who knew I wasn't supposed to get water in the empty outlet

(I had moved the rack the strip was under, and left it exposed while feeding. Totally user error...I replaced the power strip, and that Tstat is still in use today).
Those 4 - Herpstat, Helix, Ranco, Johnson Controls - are probably the widest used by serious hobbyists. The biggest factor in narrowing your choice is whether you want an on/off unit, or require a proportional Tstat. The Helix and Herpstat are proportional Tstats, but can be used in on/off mode (not sure WHY one would utilize that option, but its there). The JC and Ranco are strictly on/off.