The key features to an HTML editor, for me, are :
1) Discrete and unobtrusive code completion and hints
2) Ability to review code modifications as soon as you make them
3) Presence of debugging and productivity tools, such as a DOM inspector, color grabber etc
4) Ability to scale up (integrate with larger, more complex projects) and down (simple, one-shot projects) as needed
To that end, altough I appreciate the light weight of standalone editors such as Notepad++, I'm more inclided towards larger-scale, integrated solutions such as Netbeans and Eclipse. Because however they do have a learning curve and they are, compared to lower-end soltions, resource hogs, I'd highly recommend both the Aloha and the Maqetta editors and it's always great to have Neotepad++ around as a generic text editor.