Jiivya - Jiivya is a weird one. Rather than saying “this combo works great with Jiivya”, it’s easier to think the other way around. Jiivya gives you great regen, stats, beneficial mutations, and is overall super strong, but you lose most items in the game. That includes weapons, armour, books, consumables, everything. Slimes will eat it all. If you have a character where the positives outweigh the negatives, take Jiivya. This will be a very rare choice though because the altar doesn’t spawn much in the early game, and by the time you see one, you probably don’t want to abandon your current god.
Mahkleb - Decent on most anything, though the most recent nerf made Mahk a “don’t bother” god for me. Heal on kill, lesser/major destruction, and summoned help are all nice but also have drawbacks. Summoning a hostile demon can be really bad for example.
Qazlal - ehhh. I would never pick Qaz if I wanted to win, with a possible exception of a melee gargoyle or formicid, both using large shields. You’re more or less marked all game, but clouds and Qaz abilities are amazing. Qaz is really fun but challenging. I’d stay away from Qaz on anything not using heavy armour.
Ru - Good for any character. A benefit to Ru is that Ru has no wrath for abandonment, except you keep your sacrifices. That means you can take a D2 Ru altar, then if you find something else you would rather have, feel free to switch.
Yredelemnul - I loathe Yred. I won’t comment on what combos work well with Yred because there is literally never a situation where I would purposely worship him
So, that’s all the gods out of the way, but I wanted to point out a broader strategy to picking a god. Think of your character like a tool box. Your tools are generally melee, range, magic, consumables, innate abilities, and god abilities. Every god has their own set of tools that may benefit tools you already have (Veh makes your spells stronger for example), or give you new tools to deal with situations, etc. Whenever you see an altar, if you don’t already have a god, you should ask yourself “what tools does this god give my character? Do those tools fill in a gap that I need? Are they beneficial? Detrimental?” Then make your decision based on that. Here’s a basic hypothetical:
You’re playing a straightforward Naga Fighter. Your tool kit at the beginning is melee, poison spit, consumables, high stealth, high hp, etc. In exchange for this great tool kit, you move slowly which makes running basically impossible and repositioning needs to be done carefully. You’re thinking “man Okawaru or Trog would be great for this”, and you would be right. Now you’re on D2 and you see a Fedhas altar.
“What does Fedhas do for me? He makes plants neutral/friendly, allows you to move through plants, gives you some interesting summons, etc. Do those tools fill a gap? Well I can’t run from stuff or reposition, but if I had a plant to block enemies that will help. Ballistomycetes can help me in difficult battles, or I can use my stealth to run away while the mushrooms distract things. Do these tools benefit other tools I already have? Not really. Fedhas doesn’t give me Heroism or Berserk to make mymelee attacks stronger, but I’m a fighter so if I really need some more power for a hard fight then I have a Potion of Might. Sounds like my only negative to picking Fedhas is that I can’t pick Trog later”.
Based on that analysis, I would say Fed is a fantastic pick for this character. Is Okawaru better? Probably, but even though DCSS is quite difficult, it is extraordinarily well balanced, and just about any character with any god can win. In this case Trog might give a slightly better chance of winning, but you may not find Trog until D9, and right now Fedhas is great. I’d pick Fedhas in this situation without any hesitation.
I’m rambling a bit, hope this makes sense. I’m happen to expand on things if you’d like Happy Crawling!