Google Earth has no competition for 3D route planning, but the interface can be intimidating to new or infrequent users. These 2D sites are more intuitive for new players of the game.
Gaiagps.com was originally developed as more of a phone app than a browser route-drawing tool, but in the past five years Gaia has added significant functionality to the website. Independent of the phone app (and subscription), the website is my preferred 2D mapping site. Toggle various baselayers, look for public tracks in the area of interest, draw lines, points, etc. And of course, with a subscription, your routes are automatically synced to your phone for off-line use.
Caltopo is the only other 2D contender in my opinion. CalTopo was started by a California SAR member who wanted to provide useful information to his SAR team. Baselayer tiles load quickly, including topo, satellite, shaded relief and more. (Gaia actually serves many of the tiles generated by Caltopo.) Planning and printing can be done for free.
For my purposes, the greatest value from CalTopo is Google Earth integration and the superoverlay.kml, which makes the Caltopo layers visible in Google Earth. You need at least a basic subscription to access the superoverlay ($20/year). Sign in, click on your account name (your email address in upper left) → Your Account → KML (hyperlink at bottom).
Useful overlays include slope shading and weather stations. Right-click for elevation profiles and View from Here eye candy. You can import/export routes to/from Google Earth and GaiaGPS.
I prefer Caltopo’s printing interface over all other sites, but I still end up having to adjust contrast to get the images I want. Maps can also be exported as Geospatial PDFs, which can be viewed with phone apps like Avenza Maps.
I don’t think these do anything that Caltopo doesn’t do (better), but here are some other topo map resources: