The problem isn't the Slytherin team, it's being up against Harry.
The Slytherin Quidditch team is actually fairly good in general, either despite or because of their brutal tactics. The problem, quite simply put, is Harry messed it all up for them.
Before Harry: The Gryffindor team isn't good and lost heavily to Slytherin at least once.
Once she sees how well Harry can fly, McGonagall wants him to join the Gryffindor team, especially because their team isn't very successful.
“I shall speak to Professor Dumbledore and see if we can’t bend the first-year rule. Heaven knows, we need a better team than last year. Flattened in that last match by Slytherin, I couldn’t look Severus Snape in the face for weeks …”
*- *Harry Potter and the Philosopher's Stone, Chapter 9 (The Midnight Duel)**
In Harry's third year, Oliver Wood laments that Gryffindor hasn't won the Quidditch Cup in seven years, so that means they lost it for at least 5 years (Wood wouldn't have been at Hogwarts before that so might not be counting the time before he began) and then the two years Harry was there.
“Gryffindor haven’t won for seven years now. OK, so we’ve had the worst luck in the world – injuries – then the tournament getting called off last year …”
*- *Harry Potter and the Prisoner of Azkaban, Chapter 8 (Flight of the Fat Lady)**
Every Quidditch match at Hogwarts we know of:
Harry's first year:
Slytherin v Gryffindor (Harry's first match) - Gryffindor wins
Slytherin was ahead 40 - 20 before Harry caught the Snitch.
Gryffindor v Hufflepuff - Gryffindor wins
How much either team managed to score was unclear, but the game lasted less than five minutes as Harry caught the Snitch in record time and won the game.
Gryffindor v Ravenclaw (no Harry) - Ravenclaw wins
Harry's second year:
Slytherin v Gryffindor - Gryffindor wins
Slytherin was doing quite well in this match, scoring 60 points to Gryffindor's 0.
Gryffindor v Hufflepuff - canceled because two students were Petrified
Harry's third year:
Gryffindor v Hufflepuff - Hufflepuff wins (because Dementors made Harry panic)
Ravenclaw v Hufflepuff - Ravenclaw wins
The Ravenclaw team is described as having "flattened" Hufflepuff in this Quidditch match.
Gryffindor v Ravenclaw - Gryffindor wins
When we last heard the score before Harry catches the Snitch, Gryffindor was ahead 80 - 30. This is also the first match Harry plays with his Firebolt.
Slytherin v Gryffindor - Gryffindor wins
Gryffindor was ahead 80 - 20 in this match, with the combination of Harry as Seeker and having a Firebolt.
Harry's fourth year: Quidditch canceled because of the Triwizard Tournament.
Harry's fifth year:
Slytherin v Gryffindor - Gryffindor wins
Slytherin was ahead 40 - 10 when Harry caught the Snitch, and Draco Malfoy only narrowly missed catching it.
(Harry gets banned from Quidditch)
Gryffindor v Hufflepuff - Hufflepuff wins despite Gryffindor catching the Snitch
Hufflepuff wins 240 - 230 although Ginny, the substitute Seeker, catches the Snitch.
Slytherin v Hufflepuff - Hufflepuff wins
Hufflepuff wins narrowly over Slytherin in this game.
Gryffindor v Ravenclaw - Gryffindor wins
Ron barely scrapes by a win for Gryffindor.
Quidditch Cup: Gryffindor
Harry's sixth year:
(Since Umbridge is gone Harry isn't banned from Quidditch anymore)
Slytherin v Gryffindor - Gryffindor wins
Malfoy is off as Seeker, and a substitute called Harper takes his place. The last score we hear is 60 - 0 and Gryffindor is mentioned after that to have kept scoring. However, they couldn't have been any more than 140 points ahead of Slytherin when Harry caught the Snitch, because he knew Slytherin would win if their Seeker caught it instead. In fact, Harper did nearly catch the Snitch, but Harry taunted him and distracted him.
Gryffindor v Hufflepuff - Hufflepuff wins (because a Bludger hit Harry)
After Cormac McLaggen accidentally hits the Bludger into Harry, Hufflepuff wins 320 - 60.
Gryffindor v Ravenclaw - Gryffindor wins (despite Harry being banned)
Harry is banned from playing Quidditch as a punishment for using Sectumsempra on Draco, so Ginny substituted in for him as Seeker. Gryffindor still beats Ravenclaw 450 - 140.
Harry doesn't show up for his seventh year and we don't know about any of the matches.
Once Harry is able to play in top form, every other House team loses.
The only matches Gryffindor loses after Harry joins are the ones that Harry either couldn't attend, or was injured or otherwise unable to play well (like when the Dementors invaded the pitch). Once Harry was present and didn't get injured or panicked by Dementors while playing the match, his team won every time, no matter which House they were playing against. Slytherin actually seems to have come the closest to beating Harry.