Returning from the Immortal World is a good read read but very slow strength gain and not really a harem haven't read the other one
tempest of stellar war is a good read, but doesn't really have a harem in it. There are many girls that like the main character, but that is all. I haven't read the other novel so i can't say for sure.
Battle Frenzy - it's from the same author, almost the same, just that it's updated much faster than tsw
TSW is good as stated above, no harem~loyal to love interest (so far). I haven't read the other so i cant say..
tsw is good, more laid-back, fun story, while the other is a little bit more serious, action story, but with a slower pacing, making it a calm read.