There's no right or wrong answer to any of these, it depends on your spend patterns, and what you spend on. The giftcard loophole on the Amex BCP for instance has been popular because you can get up to 6% effective on a lot of spend, although there is a cap on the 6% cashback in eligible spend per year.
The general easiest everyday best spend card is the Citi Doublecash because no AF and 2% simple cashback, not points that can be re-valued (devalued), acceptance of a mastercard, open to basically anyone in US to apply. To my knowledge, that's the best no annual fee card available that is still taking applications (US Bank Limitless Cashback is no longer taking applications. If you can park $100K or more at Merrill or BoA, then you can get a 2.625% effective no AF card at BoFA. If you're willing to take on a huge annual fee, as mentioned earlier upthread the US Bank Altitude Reserve offers 3x points on mobile wallet at a redemption of 1.5cpp = 4.5% cashback effective. The only problem for this is, at least near me NFC (Apple Pay/Google Pay) acceptance is not that great, and some magswipe readers are still fussy with Samsung Pay. If you take the travel credits on the card as a given, then the breakeven on cashback minus remaning annual fee is $3000 (earns $60 cashback on doublecash, earns $135 on the US Bank Altitude Reserve minus $75 remaining of annual fee = $60).
The other risk, of course, is that US Bank may find that unlimited 4.5% rewards on mobile wallet is too good to last, and then cap this category cashback to a lower amount (like $1500 in eligible spend per quarter) like the US Bank Cash + (which originally earned unlimited 5% cashback in two categories you picked, hotels and rental cars included - some small business owners picked 5% on online bill pay instead). So I'm tempted to think about the Altitude Reserve, but I feel like it's something that's too good to last.
In terms of general spend, excluding specific airline/hotel credit cards:
5% rotating: Chase Freedom and Discover It (5% on rotating categories, up to $1,500 per quarter)
5% on gas: PenFed Cash Rewards Visa, no annual fee ($1,500 deposit account balance at PenFed required to waive annual fee). It earns a miserly 0.25% on all spend other than gas paid at the pump itself. With the costco card at 4%, might not be worth getting if you already have that (I got the PenFed Visa when Costco was still taking Amex)
5% on rental cars and restaurants (up to $1,500 eligible spend per quarter): US Bank Cash+, no annual fee (also there are other categories you can pick from)
4% restaurants: Uber visa (no annual fee).
3% grocery: Amex BCE (my grocery spend is not high enough to merit the BCP, because certain stores like Costco don't code as grocery and Costco doesn't take amex anyways)
2% overall: Citi Doublecash
The other balancing act is how good the benefits set is. One payout on purchase protection, extended warranty, or price protection can eclipse cashback on many purchases. Citi has some great policies on this for the Double Cash, whereas US Bank uses eclaimsline (the one claim I did with them for rental car damage coverage was a nightmare, saved only for the fact that Hertz wanted to get paid and had backdoor phone numbers/emails to sort the entire claims process with their analysts). It's another reason why I'd hesitate to get the US Bank AR card and spend it on a lot of bigger ticket purchases.