Tl;dr - Schedule and price. I have limited non-stop opportunities, so everything is a connection.
I've been flying a lot more UA this year (1K through corporate match) and it's been interesting to compare/contrast.
Ontime arrival: DL less than 50%, UA 95% (this is about 80,000 miles on DL, 75,000 miles on UA since JAN 17, but certainly has to do with airports I travel to - BOS, PHL, NYC, SFO)
Service: The worst flight attendant I've ever had was on UA. The best flight attendant I've ever had was on UA (including QF, CX, BA Int'l First). I find DL to very consistently be down the middle - I've never had excellent service on DL, but I've also never had poor. Free snack in coach for 1K is very nice and I make use of this often. Elite recognition for 1K is lightyears ahead of DL for Diamonds (but I'm guessing they're taking notice based on recent surveys)
Upgrades: DL wins hands down. Based on my travel, I get DL upgrades about 60%. UA as a 1K on an M fare PHL-ORD (instant upgrade) - I better pay my extra $149 ow, or there's no chance I'm getting a seat up front.
IRROPS: It's a wash for me. UA had been very proactive in my IRROPS and will confirm me in F even if I was on a comp upgrade. DL also treats its Diamonds well.
Club: No access to United Clubs, having SkyClub access is a godsend (and since it's a full membership as Diamond, I can use the SkyClubs when traveling through UA hubs [DEN, ORD, EWR])
Schedule: DL has more flights out of MSN, but UA has well timed flights to go West through DEN
Price: It's always a dice roll - sometimes DL is $100 cheaper, sometimes $200 more. I go with whichever carrier is cheaper to save the customer $$$ as long as the schedule is appropriate.
AA? God no. I'd rather fly Spirit.