well... the first decisiion you have to make is unix (linux or similar) vs. vms (win 2k, xp) vs. dos (win 98).
win 98 is a bad choice for more reasons than i can mention: so this leaves win vms vs. unix:
my personal choice here would be unix. since it is more configurable, has greater security options, has a tonne of open source software if you want to expand and because there is more documentation for it than for windows (at least when it comes to webhosting).
once you've decided on unix the decision comes down to linux vs. bsd vs. proprietary (ie solaris, aix). you can probably count out proprietary since it's expensive, not well documented in the public area and often has less software available (except for expensive payware).
so it's linux vs. bsd... well that's a topic for regligious zealotry so i'll avoid making a decision and just offer a suggestion in both areas:
bsd: openbsd is a good choice here. it's free and open source and has a reputation for being super secure (these are the guys who gave us open ssh and blowfish). the man pages and docs are good (better than linux man pages at least) and its ports system is better than rpm. you also get the advantage of looking like a real geek. openbsd also allows you to easily set up things like vpn's and weird routing. the big drawback is that the install is a bit challengeing.
linux: red hat is probably best here because it is the most supported. the rpm system is terrible (compared to bsd ports and apt-get at least) but it works. the big advantage of red hat is that if you ask a question on just about any linux forum you'll find way more answers for red hat than others. the install is super easy too.
both openbsd and red hat allow you to run apache, mod-ssl, php, mysql, postgresql, sshd, ftpd, perl, python, ruby and just about everything else you need to for a host.