A lot of main leagues fixtures are in a point where some teams need to play like it's their major cup final. Every lost point can mean automatically that its a bye-bye for Champions League, Europa League or can cement the relegation to a lower league.
As the most followed league is Premier League, I picked out two fixtures from there and one from German Bundesliga.
There's a lot of fixtures to be played in Premier League and of course mathematically, the league title is not handed over yet. 
In reality, Liverpool is uncatchable, and they have lost only once from 28 games. We all know it's over! There's also Manchester City, who will play in Champions League next season? Everything else is still open in my opinion.
 

Premier league key fixtures

Leicester sits on 3rd place at the moment, but it's hard to save them the CL spot already. 2 draws and two losses from last four games. Especially worrying was their last fixture against Norwich, red lantern in the league.
Wolves and Manu are in better shape, latter thanks to new winter signing who has proved quickly the quality he provides. I would say there are 3 Champions League positions available and also Europa League group stage spot. Four sweet spots and ten teams are fighting for them. The surprise teams in the mix contending for these spots are Burnley, Sheffield United and Crystal Palace. Usual suspects are Tottenham, Manu, Chelsea, Leicester and Arsenal.

Arsenal v West Ham key fixture

Arsenal has had a season to forget so far. Thirty-seven points and 10th place after 27 games. This fixture can raise them to 40 points, and they have also one game less played at the moment. If they win this game, it means we have three teams on 40 points and only 5 points from first Champions League spot.
They are on fire at the moment regarding scoring the goals. In the last two home fixtures, Arsenal has scored seven goals in total. Aubameyang has found his touch again and 20 years old Edward Nketiah is delivering. Mikel Arteta has managed to swap the lineup several times during the season and seems that he has found his group who can perform like the prefers. They have still depth on the bench, as Lacazette, Guendouzi and Torreira are eager to get back to starting lineup. Tierney and Chambers are still out, but the fight is on!
West Ham finds themselves on 27 points and 16th place. They have won only once in their last five games, but two of these games were against Liverpool and one against Man City. They managed to win their final fixture against Southampton. At home and 3-1 where their current top scorer Sebastian Haller showed a good form. West Ham is sitting on 27 points with two other teams. One of them is Bournemouth and in relegation place. So, it's a crucial game for them also. Ok, crucial is maybe overrated, but the relegation battle is tough and dropping back to Championship would be a disaster for West Ham.

Manchester United v Manchester City key fixture

To ignore Manchester derby in a key fixture blog would be a sin!
Manchester City is a heavy favourite to win the game from the odds, on away ground. Already this is an amazing fact for me as I don't remember such things if we go back one year and more. How time changes ough.
The City doesn't win the league this year; you heard this from here first!
If they will play Champions League football next season is up to UEFA. Either way, the situation is not clear and there can be extra Champions League spot on the table.
Manu situation is not so good at the moment. On a positive note, their form has picked up a lot. The loss against Burnley at Old Trafford was a disaster of course. Since Bruno Fernandes era, they have got points from 4 games, two wins and draws. In his first four games, he has already two goals and two assists, absolute key figure for them.
Both teams also have FA Cup game in the middle of the week. The City has an edge here as they have the deeper squad and even one-day extra rest before the derby and this is a big thing. It's hard to doubt about the motivation in this kind of derbies, but Citys situation at the moment can be devastating for the motivation of the players. Pep has a lot to do to rally his troops. This fixture is more critical to Manu, but the quality is on the City side, is the motivation also there?