I-5 and CA 99 are toss-ups. I-5 is a direct through route, a somewhat boring drive with fewer on and off ramps. CA 99 has more local traffic going from town to town. 99 is also an older route, it was originally US 99 before I-5 was built.
When you get to the bottom, take CA 58 over the Tehachapi Mtns. If you take I-5, CA 223 through Arvin is a good alternative to working your way over through Bakersfield. The Tehachapi route is a moderate climb to about 4,000 ft., winding through the mountains to Mojave. Beware of wind advisories, near the summit you'll pass through CA's largest wind farm and it's there for a reason. Usually you'll have a tail wind going east, but there can be crosswind gusts east of Bakersfield as you head into the mountains.
If you have the time US 395 is a longer but very scenic drive, but you have to climb over the Sierra Nevadas to get to it. I-80 is the most direct route, US 50 is slower but more scenic. Just avoid Fridays and the weekends when there's lots of cars and tour buses going to South Lake Tahoe. Along 395, be sure to stop in at Schat's Bakery in Bishop. Between Bishop and Lone Pine is the Manzanar WWII Japanese internment camp. Lone Pine has the Alabama Hills where Hollywood filmed numerous westerns.
I'd continue east on 58 to Barstow no matter which way you get there. There may be some delay at the 58/395 stoplight until they finish the bypass, other than that 58 is now 4 lane freeway the whole way. From Barstow take I-40 to Needles, then US 95 on the CA side or AZ 95 on the Arizona side south to I-10. If you use the CA side, turn east at CA 62 to Parker, then take AZ 95 south to I-10 at Quartzsite.
Or take AZ 95 south. It's a better road than US 95 on the CA side, but slightly longer and you'll have some delay going through the half dozen or so stoplights in Lake Havasu City.