Minotaur

I love Minotaur.

I'm only Epic, but my experience is the following:
Build him as a tank. Choose items that will make you more tanky.
Your absolute top priority, especially in the early game, is keeping the marksman safe and helping them to farm. They call it "roaming", but that's a misnomer: the jungler actually does a better job at roaming because he's in the jungle and can pop out into a lane to assist at any time (mid also roams better than roaming does). Meanwhile, as a roamer, you will be more helpful to your team if you stay in gold lane and protect the marksman. The roamer and marksman combine very well because the roamer has high hp and low damage, whereas the marksman has high damage and low hp. You want the enemies to attack you, while your marksman is safe behind you, raining damage upon the enemy.

The next most important priority, especially in the midgame and late-game, is supporting your allies in fights. Again, you want the enemies to be attacking you instead of more squishy targets, while your allies are doing actual damage to their squishies. The greatest thing about Minotaur specifically is his epic AOE CC, especially his ultimate! This makes him really good in team-fights, because you can keep the enemy stunned for a long time. His ultimate will refresh the cooldowns of his other abilities (and upgrade them), so you generally want to jump in, ult, then jump again. Naturally, you want to stun as many targets as possible, especially their marksman and anyone else who deals a lot of damage. His ultimate is also super powerful in the early game, and can get you some kills if the enemy is too aggressive.

You never want to be farming. You have like zero damage, and can't kill creeps, and you have roaming equipment anyway. In the mid-late game, if you can't do either of the above priorities, then the next most important priority for you to do is to move out onto the map: your purpose in doing so is to gain vision for your team, so that they know where all the enemies are, and as a result, they don't get ganked and die. But don't get close enough to the enemy for them to actually engage in combat with you; keep them on the edge of your vision, so they can't attack you, but you can see them. Because you're tanky, this is usually relatively safe, but don't be reckless in such a way that you end up being caught out and killed. In the same way, throughout the game, you should be checking bushes if you think that an enemy may be hiding there, so that your more squishy teammates (especially the marksman) don't get ambushed.
Except on the first skill-point, you should probably be leveling up your heal over your jump. You will be healing pretty much every time it comes off cooldown, assuming that there are damaged heroes nearby. You will also be jumping often, but less often, and the damage increase per skill-point on the jump isn't big enough.
One of the downsides of his ultimate is that it has a super slow wind-up. If Minotaur isn't building tanky enough items, it's sometimes possible for Minotaur to be killed before he completes it. On the other hand, his jump is super quick and will cause a small stun of the enemy nearly instantly.

Naturally you want to choose revitalize (the healing one) as your battle spell Flicker and vengeance may also be viable.
One issue with Minotaur is that you have no way to force enemies to attack you. You have invested a lot of money into building tanky items, but then the enemy might just choose to ignore you, and since you have no damage, you can't do very much to them. The main thing that stops the enemy from doing this is your epic ultimate, which provides a huge stun to help your teammates kill the enemy, as well as your jump which also provides a stun. Still, you had better use these early, before the rest of your team dies around you. Otherwise, you might find yourself a lone Minotaur with full hp surrounded by angry enemies.
At one point, I tried going for a build which would aim to maximize cooldown reduction, so that I could use my epic ultimate more often. I did this due to the realisation that the enemy could just ignore me, and target squishy heroes instead of me. However, I found that this wouldn't help, because it wouldn't reduce the cooldown by enough in order to use the ultimate twice in one team-fight. Also, the natural cooldown is usually short enough to use it in two consecutive team-fights. I found that the reduced tankiness was hurting me, so now I aim mostly for tanky items.

I personally don't think that the extra heal from flask of the oasis is worth it, I don't find that it is impactful enough. I personally go for thunder belt as my first item. However I have nothing against flask of the oasis: I think it's very possible that it's the strongest item when you get more skillful than I am. I just personally don't use it.
I also love Belerick. I initially thought that Belerick was simply better than Minotaur: Belerick often has the ability to 1v1 the enemy marksman, which I love. Belerick is a marksman eater. However, after playing both of them, I have come to appreciate each of their strengths and weaknesses. The biggest advantage of Minotaur is that his jump is a practically instant stun, even if his ultimate has a very long wind-up. Meanwhile, both of Belerick's stuns are very long wind-up, which makes them hard to hit.
Some people don't like roaming because they feel like a "support" role isn't as important and won't have a big impact on the game. This is false. A good roamer can cause a bad marksman to get a few kills, and a good roaming/marksman team is unstoppable. It's also super easy to get MVP as Minotaur: it's easy to avoid dying (since you're tanky), and since you're participating in most teamfights, you get a ton of assists. Hopefully no kills, if you are letting your marksman finish off enemies for the extra gold. I reckon MVP and KDA are both pretty misleading, though, anyway.

When you play well, and everyone is acclaiming the marksman who got 14 kills and 0 deaths, remember that it was you who made this possible. It was you who kept the marksman safe at their weakest, while they were farming. It was you who stunned the enemy so that the marksman could finish them off. It was you who caught all five enemies in your ultimate while your allies were in just the right position to take full advantage. Remember that.

It's true that it's harder for a good support to win if they are supporting a bad marksman, than it is for a good marksman to do well if they have bad support. However, even as a marksman, it's difficult to do well if you don't have good support and your opponent does.
If you feel like you did your best, and the marksman who you are doing your darndest to support is playing poorly, and you feel like all your work is going to waste: don't worry, don't give up. Keep trying your best to win the game, in spite of everything. It may well still be possible to win: sometimes, the rest of the team can carry for them, and a late game marksman can still do a lot of damage, even if they don't play very well.

Never blame another person for your loss, because this prevents you from improving. Yes, other people played badly, and maybe you played better than they did. However, you did not play perfectly: a pro player may very well have been able to turn the game around in spite of their teammates. It is always possible for you to improve, and focus on that.