To me, most enjoyable = most versatile in gold. I'll be narrowing down potential classes by adding criteria just for fun.
What you need:
1. A Paladin/Carnifex: These heavy pistols are easy to use, have serious one-shot power, excellent accuracy for long-range shooting (no need for a scope), a nice little mag so you can fire a decent amount of shots before reload-canceling, a fire rate just high enough for close encounters. Great against armor. Add a piercing mode and a heavy barrel so you can avoid AP ammo and go with one of the best two ammo types with this gun: cryo or disruptor (allows you to stagger/slow an enemy with the first two shots so you can land a headshot easily on you third shot). I strongly recommend Disruptor as you get a 4x damage multiplier against shields and barriers.
2. Self-detonating combo explosions so you can generate massive single-target and area damage, and take advantage of your low power recharge time for carrying only a pistol. I recommend Biotic Explosions. Choose a combo that can be detonated from any distance. The damage is insane, it destroys bosses, it has a multiplier against barriers (1.5?).
3. A priming power that staggers and/or debuffs. Warp does both + it deals instant damage and damage over time + it primes for a long time (you have time to empty your mag on a Phantom before you detonate and kill her.)
4. A dodging ability, a fast Heavy Melee of 1000+ damage, and either 1000+ shields or 700+ shields w/tech armor so you can do well and be aggressive in close encounters with your heavy pistol without dying.
I think that's enough to narrow things down to the Human Sentinel
Seriously, this guy can do about anything. He is a tanky jack of all trades with great mobility.
Take evo 5 Melee Damage in Tech Armor (maxed-out for damage reduction) + evo 4 Melee Damage in Fitness (then shield and fitness).
Take the detonate evos in both Warp and Throw.
Spec out of Alliance Training to max out warp and throw, or take it to 4b Damage & Capacity and take Warp/Throw to evo 5 only --totally OK since level-5 Biotic Explosions are already very powerful, and you get +20% power and force damage which is great for Warp and Throw (and compensates the fact that we went with melee damage instead of power damage in Tech Armor), and a small power recharge reduction (nice since we went with durability instead of power recharge in Tech Armor).