The problem with most time travel shows is they suck at being time travel shows.
They seem to all do one or more of the following:
1) They make up rules of their time travel universe, then violate them when the plot calls for it.
2) The characters suck at blending in. They either decide they have to change history to kill Hitler, save Lincoln, prevent Pearl Harbor, catch Jack the Ripper and prevent the sinking of the Titanic or they simply slip up with modern vernacular or revealing advanced tech to the people in the past. The worst ones are the clueless characters that shouldn't be allowed to travel without any sort of knowledge of the past.
3) They make up stupid rules that simply make no sense, like the universe will implode if you meet yourself in the past or you can't visit a time where you exist in the past. You also get the extreme ones where tracking a single leaf back on your boot causes the apocalypse or trying to save your childhood girlfriend results in you losing your limbs or having to abort yourself.
4) They use some sort of mammoth immobile time travel vessel that you could never hide and somehow manages to be brought back despite the shear amount of laboratory resources (large amounts of power, a rocket sled track, etc.) required to send it into the past.
5) In order to keep the show from being a simple "stop someone from using the past for crime or profit" story line, you have to add in some overarching conspiracy that just happens to involve one of the good guys, likely in some sort of causality loop (ie: character created "X", which causes "Y" that is exploited by the bad guys). The good guy eventually either a) prevents him or herself from, creating "X" or destroys "X" to eventually prevent "Y", which undoes every change except the time travel event itself, which should never have occurred.
Timeless managed to hit all points of the "Time Travel Evils Pentagram of Things that Make Time Travel Stories Suck
". Plus, everyone was profoundly stupid, which is why #2
In history (no pun intended), there have been a few really good examples of time travel fiction in movies and TV. Not all of them were intended to be taken seriously, but even the lighter fair has been successful by avoiding the above issues - or by mocking them in some way:
1) Back to the Future (1985): This is the Grand Daddy of comedy time travel. It stays consistent with its rules, the main character was never intended to be a time traveler (so you can forgive him not blending in), the stupid rules become a joke ("I figured, what the Hell...."), the vessel is a gas powered car with a plutonium powered time travel power plant device (which works to trap the character in the past) and the whole plot is about undoing the time traveler's own changes to the past. Further, we're shown the flux capacitor with the only explanation needed being that it makes time travel possible. The vehicle is a Delorean, which by its nature must help make time travel possible due to its unique stainless steel body. The most satisfying part is that every element we encounter in the opening scenes resolves itself through the time travel experience, yet it avoids feeling like a paradox by returning the character home after the point he left.
2) 7 Days (1998 series): This show also avoids a lot of issues by offering a very tight and reasonable premise: you can only go back 7 days and the trip is one way. The military handles the time travel missions, meaning the sphere is picked up and brought back to the super secret base. The premise is simple: a catastrophic event occurs and the team must figure out a plan to prevent it that can be put into action by the time traveler with whatever time is left of the 7 days. The only issue is the paradox of preventing the disaster, which should prevent the need for the mission, but technically almost every bit of time travel would involve a paradox, short of simply going back to watch.
3) Journeyman (2007 series): While this show does touch on the "overarching conspiracy" domain, it does it well. If there was one flaw it was that the show was cancelled far before it should have been.
4) Quantum Leap (1989 series): This is a more interesting take on time travel in the sense the character is becoming someone in the past, rather than ending up in the middle of it as himself.
5) Time After Time (1979 film, not the series): This should suck. We have another story involving trying to stop Jack the Ripper, but it becomes good when the twist is that Jack has gone on a one way trip into the future. That works in two ways: 1) we aren't trying to prevent Jack from murdering people in the past and 2) it provides a great explanation of why the killings suddenly stopped in that time period. The flaw of a time travel vehicle you can't easily hide is removed by having it show up in a museum exhibit for HG Wells.
6) The Time Machine (1960): This one also has a vehicle you can't easily move, but the plot works within that limitation. This one also removes the mistake many "future" plots make by eventually being proven wrong when we get to that date (i.e., Back to the Future II) by having the main future plot be so far into the future, none of us (or even our great, great grandchildren) will ever live to see it.
7) Star Trek IV: The Voyage Home (1986): After Star Trek II: The Wrath of Khan, this is arguably the best original cast Star Trek movie. Despite what sounds like a ridiculous plot on paper, it actually works. What's more, it's fun with many great lines.
8) Frequency (2000 movie): This movie avoids the whole issue with actually time traveling by merely having characters from two times be able to communicate through a Ham radio that has somehow connected to its past self. It shows how tinkering with the past can have unintended consequences in the future.
9) Timecrimes (Los Cronocrimenes) (2007): An excellent example of a well executed Causal Loop film.
10) Groundhog Day (1993): Often imitated, but never exceeded, this is the gold standard in time loop stories. The fact that it's a very funny comedy makes it even better.
11) Grand Tour: Disaster in Time (1992): One of the better "Time Tourist" movies.
12) Primer (2004): This one deals with not only the mechanics, but the ramifications of time travel.
13) 12 Monkeys (1995): Time traveling to stop a pandemic? That's crazy, right? Much better than the episode of The Outer Limits (Patient Zero) that came along in 2001.
I've also head good things about the TV shows Continuum and Outlander.
I'm a bit partial to "The Final Countdown", even though it involves a "let's stop Pearl Harbor" plot line. However, it finds a reasonable way to get out of the situation. I also liked "Timerider: The Adventure of Lyle Swann".