Although I love Star Trek, I don't think they have been that imaginative with their aliens. Some of that is a limited TV budget, some of it is the difficulty of doing a weekly TV show in a limited time, some of it is due to only having one person doing make-up (Michael Westmore). I'm not criticising him, he has won awards, but you won't get many new ideas out of the same person after a while.
The alien culture, heritage and societies have been well fleshed out -- look at Vulcans, Klingons, Bajorans, Cardassians, Ferengi, Romulans, Borg. They are like real people to me, and it is what Star Trek does best.
But as Ray said, they aren't very Alien; they are more like exagerated humans. For real aliens you need to look to books, where you can let your imagination fly, and there are no budget restrictions on effects.
The Star Wars aliens were more imaginative looking, but I would like much more character development of them. They were at least less like a man in a mask and suit, than Star Trek TOS and Dr.Who. However, those programmes are quite old, so for their own time they did well.
The best recent development on Star Trek was species 8472, but how long did it take them to make a cgi alien? Some say it is just a rip-off of the Babylon 5 Shadows, well there is a good reason for the similarity, after B5 finished, the graphic artists went and got jobs on Voyager and designed it. It is simptomatic to me of how Star Trek has always followed the trends, rather than set them, as Star Wars did.