Mono build armies are fun, but to be competitive you'll need to mix together a few different codex's. The most common options are:
1) Guard CP battalion. Basicially either two company commander or a company commander and Primaris psyker and 3 infantry squads, sometimes these have mortars in. Generally the Cadian doctrine, though Catachan is currently very popular. Cadian for shooting and Catachan for muscle. Other regiments are all viable.
1b) People also add in various this to this formation depending on points. Common add on's are HWT's with mortars, Bullgryns and Hellhounds.
2) Assassins, you can take one for -1CP as a Aux detachment or Three as a vanguard detachment for 0CPs. Culexus are good due to the way the psychic power smite works and for being hard to hit. Callidus and eversor next. They can infiltrate so good for grabbing objective. Also cheap in points
3) Super heavy with knight, ubiquitous ATM. Mostly Castallans and Arminger little knights
4) Smashguinius - 3 Blood angle captains with jump packs, thunderhammers and storm shield as a Supreme command detachment for +1CP or 2 with 3 units of scouts as a battalion for +5CP. scouts are really good for board control and screening due to their deployment option.
4b) this used to be 3 custodian shield captains on jet bikes as supreme command
Spacemarines, deathwatch, grey knights all have good units but overall not top tier. Dark angles are okay, black knights are good and they have good flyers and -1 to hit vehicles, along with good plasma weapons. Space wolves are new out but seem okay, a few combo's. Thunderwolf cavalry and wulfen are very good.
All of these are fairly competitive approach to list building. However, if you want a cool modelling project and a fluffy army you can take whatever you want and which ever models you like most. The list building is easy from there.