Every time I see Picard or Riker or anyone else tug on their shirt when they stand up or start walking, it makes me want to stand up and pull my shirt down.
I think every Starfleet uniform costume is uncomfortable to some degree, especially the jumpsuit ones which tend to be very tight. I remember reading somewhere that the early DS9/VOY uniforms were the worst (tight around the shoulders and prone to giving the actors atomic wedgies when sitting down).
On the other hand, Colm Meaney (Chief O'Brien) is said to have prefered them, having had long experience with the various (apparently even less comfortable) TNG versions, and seeing the early DS9 ones as being relatively less restrictive in those particular areas. I remember reading somewhere, and please someone correct me if I'm wrong, that the DS9/VOY versions and the First Contact ones were likely built in 'hero' variants. In other words, they look quite bulky, like they've got several layers underneath etc, but more likely than not the costumes may have been sewn with false undershirts into the collars etc, to make them easier for the actors to act in. Only when the action in a script actually required a character to take off the top shirt was a "full" version substituted.
I remember an interview with Patrick Stewart where he described having back problems from how tight the early TNG uniforms were. In fact I think he took some kind of labor action to get them to change.
I think the most comfortable uniforms that have ever been made for a Trek production have to be the "aboard-ship" uniforms from '09 and Into Darkness. They appear to be just military-style boots, black trousers, a black under-shirt, and the colored uniform tops. They look like the only uncomfortable thing with them is when the actors put on their utility belts. http://images1.wikia.nocookie.net/__cb20121213193906/memoryalpha/en/images/c/c0/Operations_uniform%2C_2258.jpg
It was (iirc) the elastic nature of the material the costume were made of, they constantly pulled downward on the actors shoulders. It took a slight but ongoing effort just to stand up straight. Like wearing a full backpack all day.
That's pretty much how I dress for work every day, if you substitute a normal pullover shirt, denim jeans, and steel toed boots. Hot sometimes, but it doesn't get more comfortable than that.
By the 24th century, we'll probably have elastic materials that fit like a second skin and are so unobtrusive, you'll feel as if you're barely wearing anything at all. Unfortunately, the costumers of Trek TNG had to work with late-20th-century fabrics.
The same thing was true for the ENT uniforms. The black undershirt was more or less a sleeveless bib (Montgomery was once interviewed while wearing it between takes). IIRC, Bruce Greenwood hated the original Star Trek XI uniforms (the original trousers were heavy dark brownish-gray biker pants that he found not very comfortable). In Star Trek XII, they were replaced with more conventional dark gray trousers made of a lighter material.
When I used to do a bit of acting fifteen years ago now, it was always standard practice in the theater for the costumes, especially the more elaborate ones, to be sewn to look good but not be terribly practical beyond simple wearability. For example, I once played a gunslinger in a play, and the costume was a one-piece that looked like the full thing. It looked like I was wearing all the regalia: the vest, undershirt, chaps etc, but in reality it was all sewn into this one-piece that simply slipped on over my regular clothes. The only 'practical' part of the costume was a duster jacket that was worn over the top of the one-piece (which also helped disguise that it wasn't a full outfit). Smoke and mirrors, the power of illusion, did the rest in the minds of the audience. TV and movies of course have got the added complication of having to stand up to the scrutiny of the screen resolution. I can imagine the TNG season one uniforms were considered quite practical by the production team, because what you saw was what you got and there wasn't any need to 'hide' anything from the camera... of course, the production team weren't the ones who had to wear the darn things for 18 hours every day!
The TNG ones especially the first two seasons have to rank the least favorable. When I was a kid I used to think about how in the world did they make it to the rest room having to zip the back of that down.
They had front zippers. The TNG uniforms from season three onwards, though, they had back zippers. "In-universe," however, all those uniforms probably had magic seams.
The first TNG uniforms were made from Spandex. Jonathan Frakes said in an interview that besides being uncomfortable, they tended to hold body odor and smelled bad. The later jacket/trousers variation was a wool blend. The jackets rode up slightly when sitting, which is how Stewart improvised the "Picard Maneuver".
I participated in Star Trek: The Adventure at Universal Studios back in the day and had to wear Klingon armor. The armor was just a cheaper, stripped down version of the armor from the movies and tv shows and you know what? It was still pretty goddamn clunky and awkward to wear. I don't envy the actors who had to wear the heavy stuff as well as all that goddamned makeup! Musta been hotter than hell for the poor bastards who played Klingons.