What OS Is Best

an interesting comparison of System 1 and 7

• 68000 Mac (Plus, SE, Portable, Classic) - 6.08. 7 or up is just too slow. You can find Photoshop, Illustrator, PageMaker, Quark, Director for system 6. Try thrift stores, eBay, 1/2 Price Books.
• 020, 030 Macs with 8 megs of ram or less - 7.0.1
• 020, 030 Macs with 16 or more megs of ram - 7.5.5
• 040 Macs - 7.5.5
• PowerMac with 16 to 32 megs - 7.5.5
• PowerMac with 32 to 64 megs- 8.5
• PowerMac with 64 megs to 128 - 9.x
• G3 PowerMac with 128 or more (256 recommended. Apple says you need a Mac with built in USB to run 10.3 Panther) - X

Some apps require 7.6 (internet mostly), otherwise there really is no need to run 7.6.x. I wouldn't run 8 on an 040 or PowerMac, I'd run 7.5.5 on the 040 and 8.6.1 on the PowerMac unless you have 64 megs of ram or more in which case I'd run 9.x. If you have a G3 with 256 megs or more, then X will run fine. As you can see, I see no reason to run 7.1 or 7.6 on any Mac. 7.5.3 is free as well as the upgrade to 7.5.5. Another advantage to running 7.5.3 or up is you don't have to worry about those pesky system enablers for 7.1. All the enablers are built into 7.5.3. 

System Software prior to 7.1
System Software after 7.1 and prior to 8
And of course there is always Linux...

Where to get Mac OS

6.0.3 from Apple
6.0.5 from Apple
6.0.8 from Apple
7.0 from Apple
7.0.1 from Apple
7.5.3 from Apple
7.5.5 update from Apple (add this to 7.5.3)

The above are the only versions available from Apple.

System 7.1 is not available for free from Apple due to licensing restricitons. You really don't need it anyway as 7.5.3, 7.5.5 is faster and more stable and has lots more features.

Versions after 7.5.5 are not yet available for free.

About the Early Mac OS
How to install Mac OS
Running 8.5 on 68k Macs
How to put more than one OS on your Mac
Apple recommends 7.5.5 on non PowerMacs